Gaps in the reference count btree are also significant -- for these
regions, there must not be any overlapping reverse mappings.  We don't
currently check this, so make the refcount scrubber more complete.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>