xfs: fix double unlock in xfs_swap_extents()

Regreesion from commit ef8f7fc5, which rearranged the code in
xfs_swap_extents() leading to double unlock of xfs inode ilock.
That resulted in xfs_fsr deadlocking itself on platforms, which
don't handle double unlock of rw_semaphore nicely. It caused the
count go negative, which represents the write holder, without
really having one. ia64 is one of the platforms where deadlock
was easily reproduced and the fix was tested.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Felix Blyakher <felixb@sgi.com>