xfs: move all scrub input checking to xfs_scrub_validate

There were ad-hoc checks for some scrub types but not others;
mark each scrub type with ... it's type, and use that to validate
the allowed and/or required input fields.

Moving these checks out of xfs_scrub_setup_ag_header makes it
a thin wrapper, so unwrap it in the process.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[darrick: add xfs_ prefix to enum, check scrub args after checking type]
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>