flow_offload: include linux/kernel.h from flow_offload.h

flow_stats_update() uses max_t, so ensure we have that defined.
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/flow_offload.h

 #ifndef _NET_FLOW_OFFLOAD_H
 #define _NET_FLOW_OFFLOAD_H
 
+#include <linux/kernel.h>
 #include <net/flow_dissector.h>
 
 struct flow_match {