netfilter: nf_ct_h323: do not re-activate already expired timer

Commit 96d1327a ("netfilter: h323: Use mod_timer instead of
set_expect_timeout") just simplify the source codes
    if (!del_timer(&exp->timeout))
        return 0;
    add_timer(&exp->timeout);
to mod_timer(&exp->timeout, jiffies + info->timeout * HZ);

This is not correct, and introduce a race codition:
    CPU0                     CPU1
     -                     timer expire
  process_rcf              expectation_timed_out
  lock(exp_lock)              -
  find_exp                 waiting exp_lock...
  re-activate timer!!      waiting exp_lock...
  unlock(exp_lock)         lock(exp_lock)
     -                     unlink expect
     -                     free(expect)
     -                     unlock(exp_lock)
So when the timer expires again, we will access the memory that
was already freed.

Replace mod_timer with mod_timer_pending here to fix this problem.

Fixes: 96d1327a ("netfilter: h323: Use mod_timer instead of set_expect_timeout")
Cc: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_h323_main.c

@@ -1473,7 +1473,8 @@ static int process_rcf(struct sk_buff *skb, struct nf_conn *ct,
 				 "timeout to %u seconds for",
 				 info->timeout);
 			nf_ct_dump_tuple(&exp->tuple);
-			mod_timer(&exp->timeout, jiffies + info->timeout * HZ);
+			mod_timer_pending(&exp->timeout,
+					  jiffies + info->timeout * HZ);
 		}
 		spin_unlock_bh(&nf_conntrack_expect_lock);
 	}