flow dissector: check if arp_eth is null rather than arp

arp is being checked instead of arp_eth to see if the call to
__skb_header_pointer failed. Fix this by checking arp_eth is
null instead of arp.   Also fix to use length hlen rather than
hlen - sizeof(_arp); thanks to Eric Dumazet for spotting
this latter issue.

CoverityScan CID#1396428 ("Logically dead code") on 2nd
arp comparison (which should be arp_eth instead).

Fixes: commit 55733350e5e8b70c5 ("flow disector: ARP support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/flow_dissector.c

@@ -407,9 +407,9 @@ mpls:
 
 		arp_eth = __skb_header_pointer(skb, nhoff + sizeof(_arp),
 					       sizeof(_arp_eth), data,
-					       hlen - sizeof(_arp),
+					       hlen,
 					       &_arp_eth);
-		if (!arp)
+		if (!arp_eth)
 			goto out_bad;
 
 		if (dissector_uses_key(flow_dissector,