xfs: cleanup xfs_ilock_iocb_for_write

Move the relock path out of the straight line and add a comment
explaining why it exists.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>