net: replace __this_cpu_inc in route.c with raw_cpu_inc

The RT_CACHE_STAT_INC macro triggers the new preemption checks
for __this_cpu ops.

I do not see any other synchronization that would allow the use of a
__this_cpu operation here however in commit dbd2915c ("[IPV4]:
RT_CACHE_STAT_INC() warning fix") Andrew justifies the use of
raw_smp_processor_id() here because "we do not care" about races.  In
the past we agreed that the price of disabling interrupts here to get
consistent counters would be too high.  These counters may be inaccurate
due to race conditions.

The use of __this_cpu op improves the situation already from what commit
dbd2915c did since the single instruction emitted on x86 does not
allow the race to occur anymore.  However, non x86 platforms could still
experience a race here.

Trace:

  __this_cpu_add operation in preemptible [00000000] code: avahi-daemon/1193
  caller is __this_cpu_preempt_check+0x38/0x60
  CPU: 1 PID: 1193 Comm: avahi-daemon Tainted: GF            3.12.0-rc4+ #187
  Call Trace:
    check_preemption_disabled+0xec/0x110
    __this_cpu_preempt_check+0x38/0x60
    __ip_route_output_key+0x575/0x8c0
    ip_route_output_flow+0x27/0x70
    udp_sendmsg+0x825/0xa20
    inet_sendmsg+0x85/0xc0
    sock_sendmsg+0x9c/0xd0
    ___sys_sendmsg+0x37c/0x390
    __sys_sendmsg+0x49/0x90
    SyS_sendmsg+0x12/0x20
    tracesys+0xe1/0xe6
Signed-off-by: NChristoph Lameter <cl@linux.com>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Acked-by: NIngo Molnar <mingo@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

net/ipv4/route.c

@@ -188,7 +188,7 @@ const __u8 ip_tos2prio[16] = {
 EXPORT_SYMBOL(ip_tos2prio);
 
 static DEFINE_PER_CPU(struct rt_cache_stat, rt_cache_stat);
-#define RT_CACHE_STAT_INC(field) __this_cpu_inc(rt_cache_stat.field)
+#define RT_CACHE_STAT_INC(field) raw_cpu_inc(rt_cache_stat.field)
 
 #ifdef CONFIG_PROC_FS
 static void *rt_cache_seq_start(struct seq_file *seq, loff_t *pos)