Now that the page fault handler has been refactored, the only caller
of xfs_ilock_for_write_fault is simple enough and calls it
unconditionally.  Fold the logic and expand the comments explaining it.

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>