xfs: convert scrub type flags to unsigned.

5.18 w/ std=gnu11 compiled with gcc-5 wants flags stored in unsigned
fields to be unsigned.

This touches xfs_fs.h so affects the user API, but the user API
fields are also unsigned so the flags should really be unsigned,
too.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>