xfs: refactor testing if a particular dquot is being enforced

Create a small helper to test if enforcement is enabled for a
given incore dquot and replace the open-code logic testing.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>