[IPV4]: RT_CACHE_STAT_INC() warning fix

BUG: using smp_processor_id() in preemptible [00000001] code: rpc.statd/2408

And it _is_ a bug, but I guess we don't care enough to add preempt_disable().

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>