net/ipv4/igmp.c: Replace rcu_dereference() with rcu_access_pointer()

The "rcu_dereference()" call is used directly in a condition.
Since its return value is never dereferenced it is recommended to use
"rcu_access_pointer()" instead of "rcu_dereference()".
Therefore, this patch makes the replacement.

The following Coccinelle semantic patch was used:
@@
@@

(
 if(
 (<+...
- rcu_dereference
+ rcu_access_pointer
  (...)
  ...+>)) {...}
|
 while(
 (<+...
- rcu_dereference
+ rcu_access_pointer
  (...)
  ...+>)) {...}
)
Signed-off-by: NAndreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -2539,7 +2539,7 @@ static int igmp_mc_seq_show(struct seq_file *seq, void *v)
 		querier = "NONE";
 #endif
 
-		if (rcu_dereference(state->in_dev->mc_list) == im) {
+		if (rcu_access_pointer(state->in_dev->mc_list) == im) {
 			seq_printf(seq, "%d\t%-10s: %5d %7s\n",
 				   state->dev->ifindex, state->dev->name, state->in_dev->mc_count, querier);
 		}