xfs: use file_modified() helper

Note that by using the helper, the order of calling file_remove_privs()
after file_update_mtime() in xfs_file_aio_write_checks() has changed.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>