flow dissector: correct size of storage for ARP

The last argument to __skb_header_pointer() should be a buffer large
enough to store struct arphdr. This can be a pointer to a struct arphdr
structure. The code was previously using a pointer to a pointer to
struct arphdr.

By my counting the storage available both before and after is 8 bytes on
x86_64.

Fixes: 55733350e5 ("flow disector: ARP support")
Reported-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/flow_dissector.c

@@ -390,7 +390,7 @@ mpls:
 			unsigned char ar_tip[4];
 		} *arp_eth, _arp_eth;
 		const struct arphdr *arp;
-		struct arphdr *_arp;
+		struct arphdr _arp;
 
 		arp = __skb_header_pointer(skb, nhoff, sizeof(_arp), data,
 					   hlen, &_arp);