ss: do not bindly dump two families

ss currently dumps IPv4 sockets, then IPv6 sockets from the kernel,
even if -4 or -6 option was given. Filtering in user space then has to
drop all sockets of wrong family. Such a waste of time...

Before :

$ time ss -tn -4 | wc -l
251659

real	0m1.241s
user	0m0.423s
sys	0m0.806s

After:

$ time ss -tn -4 | wc -l
251672

real	0m0.779s
user	0m0.412s
sys	0m0.386s
Signed-off-by: Eric Dumazet <edumazet@google.com>

misc/ss.c

@@ -2199,6 +2199,8 @@ static int inet_show_netlink(struct filter *f, FILE *dump_fp, int protocol)
 		return -1;
 	rth.dump = MAGIC_SEQ;
 	rth.dump_fp = dump_fp;
+	if (preferred_family == PF_INET6)
+		family = PF_INET6;
 
 again:
 	if ((err = sockdiag_send(family, rth.fd, protocol, f)))
@@ -2211,7 +2213,7 @@ again:
 		}
 		goto Exit;
 	}
-	if (family == PF_INET) {
+	if (family == PF_INET && preferred_family != PF_INET) {
 		family = PF_INET6;
 		goto again;
 	}