arp: fixed error in a comment

the is_garp code deals just with gratuitous ARP packets, not every
unsolicited packet.

This patch is a result of a discussion in netdev:
http://marc.info/?l=linux-netdev&m=149506354216994

Suggested-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Ihar Hrachyshka <ihrachys@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/arp.c

@@ -846,7 +846,7 @@ static int arp_process(struct net *net, struct sock *sk, struct sk_buff *skb)
 		 */
 		is_garp = tip == sip && addr_type == RTN_UNICAST;
 
-		/* Unsolicited ARP _replies_ also require target hwaddr to be
+		/* Gratuitous ARP _replies_ also require target hwaddr to be
 		 * the same as source.
 		 */
 		if (is_garp && arp->ar_op == htons(ARPOP_REPLY))