xfs: cross-reference rmap records with free space btrees

Strengthen the rmap btree record checker a little more by comparing
OWN_AG reverse mappings against the free space btrees, the rmap btree,
and the AGFL.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>