quota: missing lock in dqcache_shrink_scan()

Commit 1ab6c499 (fs: convert fs shrinkers to new scan/count API)
accidentally removed locking from quota shrinker. Fix it -
dqcache_shrink_scan() should use dq_list_lock to protect the
scan on free_dquots list.

CC: stable@vger.kernel.org
Fixes: 1ab6c499


Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>