xfs: refactor XFS_QMOPT_DQNEXT out of existence

There's only one caller of DQNEXT and its semantics can be moved into a
separate function, so create the function and get rid of the flag.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>