xfs: add a lock class for group/project dquots

We can have both a user and a group/project dquot locked at the same time,
as long as the user dquot is locked first.  Tell lockdep about that fact
by making the group/project dquots a different lock class.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>