xfs_db: refactor multi-fsb object detection decision making

xfs_db: refactor multi-fsb object detection decision making

Pull the "is this a multi-fsb object" decision into a separate function
that we can keep close to the actual multi-fsb object dispatcher.  We
will soon make the machinery more complex so we do this to avoid having
a big hairy if statement.

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