md: be extra careful not to take a reference to a Faulty device.

It is important that we never increment rdev->nr_pending on a Faulty
device as ->hot_remove_disk() assumes that once the Faulty flag is visible
no code will take a new reference.

Some places take a new reference after only check In_sync.  This should
be safe as the two are changed together.  However to make the code more
obviously safe, add checks for 'Faulty' as well.

Note: the actual rule is:
  Never increment nr_pending if  Faulty is set and Blocked is clear,
  never clear Faulty, and never set Blocked without holding a reference
  through nr_pending.

fix build error (Shaohua)
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

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