xfs: remove experimental tag for reverse mapping

Reverse mapping has had a while to soak, so remove the experimental tag.
Now that we've landed space metadata cross-referencing in scrub, the
feature actually has a purpose.

Reject rmap filesystems with an rt device until the code to support it
is actually implemented.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>