[XFS] stop using file_update_time

xfs_ichtime updates the xfs_inode and Linux inode timestamps just fine, no
need to call file_update_time and then copy the values over to the XFS
inode. The only additional thing in file_update_time are checks not
applicable to the write path.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31829a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>