quota: add missing lock into __dquot_transfer()

Lock dq_dqb_lock around dquot_decr_inodes()
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 7b9ca4c6 ("quota: Reduce contention on dq_data_lock")
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/dquot.c

@@ -1980,7 +1980,9 @@ int __dquot_transfer(struct inode *inode, struct dquot **transfer_to)
 		ret = dquot_add_space(transfer_to[cnt], cur_space, rsv_space, 0,
 				      &warn_to[cnt]);
 		if (ret) {
+			spin_lock(&transfer_to[cnt]->dq_dqb_lock);
 			dquot_decr_inodes(transfer_to[cnt], inode_usage);
+			spin_unlock(&transfer_to[cnt]->dq_dqb_lock);
 			goto over_quota;
 		}
 	}