icmp: Consolidate icmp_sock_handler() with icmpv6_sock_handler()

Currently we have separate handlers for ICMP and ICMPv6 ping replies.
Although there are a number of points of difference, with some creative
refactoring we can combine these together sensibly.  Although it doesn't
save a vast amount of code, it does make it clearer that we're performing
basically the same steps for each case.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

passt.c

@@ -392,10 +392,10 @@ loop:
 			udp_sock_handler(&c, ref, eventmask, &now);
 			break;
 		case EPOLL_TYPE_ICMP:
-			icmp_sock_handler(&c, ref);
+			icmp_sock_handler(&c, AF_INET, ref);
 			break;
 		case EPOLL_TYPE_ICMPV6:
-			icmpv6_sock_handler(&c, ref);
+			icmp_sock_handler(&c, AF_INET6, ref);
 			break;
 		default:
 			/* Can't happen */