xfs: rename the ondisk dquot d_flags to d_type

The ondisk dquot stores the quota record type in the flags field.
Rename this field to d_type to make the _type relationship between the
ondisk and incore dquot more obvious.

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