xfs: add scrub cross-referencing helpers for the rmap btrees

Source kernel commit: ed7c52d4bf92ac1f05b8c251a44a8bf4688f8786

Add a couple of functions to the rmap btrees that will be used
to cross-reference metadata against the rmapbt.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>