ipv4: do not use this_cpu_ptr() in preemptible context

this_cpu_ptr() in preemptible context is generally bad

Sep 22 05:05:55 br kernel: [   94.608310] BUG: using smp_processor_id()
in
preemptible [00000000] code: ip/2261
Sep 22 05:05:55 br kernel: [   94.608316] caller is
tunnel_dst_set.isra.28+0x20/0x60 [ip_tunnel]
Sep 22 05:05:55 br kernel: [   94.608319] CPU: 3 PID: 2261 Comm: ip Not
tainted
3.17.0-rc5 #82

We can simply use raw_cpu_ptr(), as preemption is safe in these
contexts.

Should fix https://bugzilla.kernel.org/show_bug.cgi?id=84991

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Joe <joe9mail@gmail.com>
Fixes: 9a4aa9af447f ("ipv4: Use percpu Cache route in IP tunnels")
Acked-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -79,10 +79,10 @@ static void __tunnel_dst_set(struct ip_tunnel_dst *idst,
 	idst->saddr = saddr;
 }
 
-static void tunnel_dst_set(struct ip_tunnel *t,
+static noinline void tunnel_dst_set(struct ip_tunnel *t,
 			   struct dst_entry *dst, __be32 saddr)
 {
-	__tunnel_dst_set(this_cpu_ptr(t->dst_cache), dst, saddr);
+	__tunnel_dst_set(raw_cpu_ptr(t->dst_cache), dst, saddr);
 }
 
 static void tunnel_dst_reset(struct ip_tunnel *t)
@@ -106,7 +106,7 @@ static struct rtable *tunnel_rtable_get(struct ip_tunnel *t,
 	struct dst_entry *dst;
 
 	rcu_read_lock();
-	idst = this_cpu_ptr(t->dst_cache);
+	idst = raw_cpu_ptr(t->dst_cache);
 	dst = rcu_dereference(idst->dst);
 	if (dst && !atomic_inc_not_zero(&dst->__refcnt))
 		dst = NULL;