xfs: improve ondisk dquot flags checking

Create an XFS_DQTYPE_ANY mask for ondisk dquots flags, and use that to
ensure that we never accept any garbage flags when we're loading dquots.
While we're at it, restructure the quota type flag checking to use the
proper masking.

Note that I plan to add y2038 support soon, which will require a new
xfs_dqtype_t flag for extended timestamp support, hence all the work to
make the type masking work correctly.

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