xfs: stop using q_core.d_flags in the quota code

Use the incore dq_flags to figure out the dquot type.  This is the first
step towards removing xfs_disk_dquot from the incore dquot.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>