xfs: Use iomap_dio_rw to wait for unaligned direct IO

Use iomap_dio_rw() to wait for unaligned direct IO instead of opencoding
the wait.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>