Btrfs: kill invalid ASSERT() in process_all_refs()

Suppose you have the following tree in snap1 on a file system mounted with -o
inode_cache so that inode numbers are recycled

└── [    258]  a
    └── [    257]  b

and then you remove b, rename a to c, and then re-create b in c so you have the
following tree

└── [    258]  c
    └── [    257]  b

and then you try to do an incremental send you will hit

ASSERT(pending_move == 0);

in process_all_refs().  This is because we assume that any recycling of inodes
will not have a pending change in our path, which isn't the case.  This is the
case for the DELETE side, since we want to remove the old file using the old
path, but on the create side we could have a pending move and need to do the
normal pending rename dance.  So remove this ASSERT() and put a comment about
why we ignore pending_move.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>