xfs: teach scrub to check for adjacent bmaps when rmap larger than bmap

When scrub is checking file fork mappings against rmap records and
the rmap record starts before or ends after the bmap record, check the
adjacent bmap records to make sure that they're adjacent to the one
we're checking.  This helps us to detect cases where the rmaps cover
territory that the bmaps do not.

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