gre: Initialise rtnl_link tunnel parameters properly

Brown paper bag error of calling memset with sizeof(p) instead
of sizeof(*p).
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -1345,7 +1345,7 @@ out:
 static void ipgre_netlink_parms(struct nlattr *data[],
 				struct ip_tunnel_parm *parms)
 {
-	memset(parms, 0, sizeof(parms));
+	memset(parms, 0, sizeof(*parms));
 
 	parms->iph.protocol = IPPROTO_GRE;