xfs: add a separate lock class for the per-mount list of dquots

We can have both a a quota hash chain and the per-mount list locked at
the same time.  But given that both use the same struct dqhash as list
head we have to tell lockdep that they are different lock classes.

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