phonet: Sort out initiailziation and cleanup code.

Recently an oops was reported in phonet if there was a failure during
network namespace creation.

[  163.733755] ------------[ cut here ]------------
[  163.734501] kernel BUG at include/net/netns/generic.h:45!
[  163.734501] invalid opcode: 0000 [#1] PREEMPT SMP
[  163.734501] CPU 2
[  163.734501] Pid: 19145, comm: trinity Tainted: G        W 3.4.0-rc1-next-20120405-sasha-dirty #57
[  163.734501] RIP: 0010:[<ffffffff824d6062>]  [<ffffffff824d6062>] phonet_pernet+0x182/0x1a0
[  163.734501] RSP: 0018:ffff8800674d5ca8  EFLAGS: 00010246
[  163.734501] RAX: 000000003fffffff RBX: 0000000000000000 RCX: ffff8800678c88d8
[  163.734501] RDX: 00000000003f4000 RSI: ffff8800678c8910 RDI: 0000000000000282
[  163.734501] RBP: ffff8800674d5cc8 R08: 0000000000000000 R09: 0000000000000000
[  163.734501] R10: 0000000000000000 R11: 0000000000000000 R12: ffff880068bec920
[  163.734501] R13: ffffffff836b90c0 R14: 0000000000000000 R15: 0000000000000000
[  163.734501] FS:  00007f055e8de700(0000) GS:ffff88007d000000(0000) knlGS:0000000000000000
[  163.734501] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  163.734501] CR2: 00007f055e6bb518 CR3: 0000000070c16000 CR4: 00000000000406e0
[  163.734501] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  163.734501] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  163.734501] Process trinity (pid: 19145, threadinfo ffff8800674d4000, task ffff8800678c8000)
[  163.734501] Stack:
[  163.734501]  ffffffff824d5f00 ffffffff810e2ec1 ffff880067ae0000 00000000ffffffd4
[  163.734501]  ffff8800674d5cf8 ffffffff824d667a ffff880067ae0000 00000000ffffffd4
[  163.734501]  ffffffff836b90c0 0000000000000000 ffff8800674d5d18 ffffffff824d707d
[  163.734501] Call Trace:
[  163.734501]  [<ffffffff824d5f00>] ? phonet_pernet+0x20/0x1a0
[  163.734501]  [<ffffffff810e2ec1>] ? get_parent_ip+0x11/0x50
[  163.734501]  [<ffffffff824d667a>] phonet_device_destroy+0x1a/0x100
[  163.734501]  [<ffffffff824d707d>] phonet_device_notify+0x3d/0x50
[  163.734501]  [<ffffffff810dd96e>] notifier_call_chain+0xee/0x130
[  163.734501]  [<ffffffff810dd9d1>] raw_notifier_call_chain+0x11/0x20
[  163.734501]  [<ffffffff821cce12>] call_netdevice_notifiers+0x52/0x60
[  163.734501]  [<ffffffff821cd235>] rollback_registered_many+0x185/0x270
[  163.734501]  [<ffffffff821cd334>] unregister_netdevice_many+0x14/0x60
[  163.734501]  [<ffffffff823123e3>] ipip_exit_net+0x1b3/0x1d0
[  163.734501]  [<ffffffff82312230>] ? ipip_rcv+0x420/0x420
[  163.734501]  [<ffffffff821c8515>] ops_exit_list+0x35/0x70
[  163.734501]  [<ffffffff821c911b>] setup_net+0xab/0xe0
[  163.734501]  [<ffffffff821c9416>] copy_net_ns+0x76/0x100
[  163.734501]  [<ffffffff810dc92b>] create_new_namespaces+0xfb/0x190
[  163.734501]  [<ffffffff810dca21>] unshare_nsproxy_namespaces+0x61/0x80
[  163.734501]  [<ffffffff810afd1f>] sys_unshare+0xff/0x290
[  163.734501]  [<ffffffff8187622e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[  163.734501]  [<ffffffff82665539>] system_call_fastpath+0x16/0x1b
[  163.734501] Code: e0 c3 fe 66 0f 1f 44 00 00 48 c7 c2 40 60 4d 82 be 01 00 00 00 48 c7 c7 80 d1 23 83 e8 48 2a c4 fe e8 73 06 c8 fe 48 85 db 75 0e <0f> 0b 0f 1f 40 00 eb fe 66 0f 1f 44 00 00 48 83 c4 10 48 89 d8
[  163.734501] RIP  [<ffffffff824d6062>] phonet_pernet+0x182/0x1a0
[  163.734501]  RSP <ffff8800674d5ca8>
[  163.861289] ---[ end trace fb5615826c548066 ]---

After investigation it turns out there were two issues.
1) Phonet was not implementing network devices but was using register_pernet_device
   instead of register_pernet_subsys.

   This was allowing there to be cases when phonenet was not initialized and
   the phonet net_generic was not set for a network namespace when network
   device events were being reported on the netdevice_notifier for a network
   namespace leading to the oops above.

2) phonet_exit_net was implementing a confusing and special case of handling all
   network devices from going away that it was hard to see was correct, and would
   only occur when the phonet module was removed.

   Now that unregister_netdevice_notifier has been modified to synthesize unregistration
   events for the network devices that are extant when called this confusing special
   case in phonet_exit_net is no longer needed.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>