xfs: simplify xfs_qm_dqattach_grouphint

No need to play games with the qlock now that the freelist lock nests inside
it.  Also clean up various outdated comments.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>