[NETNS]: Fix get_net_ns_by_pid

The pid namespace patches changed the semantics of
find_task_by_pid without breaking the compile resulting
in get_net_ns_by_pid doing the wrong thing.

So switch to using the intended find_task_by_vpid.

Combined with Denis' earlier patch to make netlink traffic
fully synchronous the inadvertent race I introduced with
accessing current is actually removed.
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -742,7 +742,7 @@ static struct net *get_net_ns_by_pid(pid_t pid)
 	/* Lookup the network namespace */
 	net = ERR_PTR(-ESRCH);
 	rcu_read_lock();
-	tsk = find_task_by_pid(pid);
+	tsk = find_task_by_vpid(pid);
 	if (tsk) {
 		struct nsproxy *nsproxy;
 		nsproxy = task_nsproxy(tsk);