xfs: create xfs_dqtype_t to represent quota types

Create a new type (xfs_dqtype_t) to represent the type of an incore
dquot (user, group, project, or none).  Rename the incore dquot's
dq_flags field to q_type.

This allows us to replace all the "uint type" arguments to the quota
functions with "xfs_dqtype_t type", to make it obvious when we're
passing a quota type argument into a function.

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