sk-inet: save the socket option IPV6_V6ONLY (v2)

Most part of services (ssh, httpd, ...) create two separate sockets
one for ipv4 and one for ipv6. If IPV6_V6ONLY isn't dumped, bind() returns
EADDRINUSE

v2: use do_dump_opt and initialize yes = 1
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

protobuf/sk-inet.proto

@@ -18,4 +18,5 @@ message inet_sk_entry {
 
 	required fown_entry		fown		= 13;
 	required sk_opts_entry		opts		= 14;
+	optional bool			v6only		= 15;
 }

sk-inet.c

@@ -204,8 +204,17 @@ static int dump_one_inet_fd(int lfd, u32 id, const struct fd_parms *p)
 	ie.n_src_addr = PB_ALEN_INET;
 	ie.n_dst_addr = PB_ALEN_INET;
 	if (ie.family == AF_INET6) {
+		int val;
+
 		ie.n_src_addr = PB_ALEN_INET6;
 		ie.n_dst_addr = PB_ALEN_INET6;
+
+		ret = dump_opt(lfd, SOL_IPV6, IPV6_V6ONLY, &val);
+		if (ret < 0)
+			goto err;
+
+		ie.v6only = val ? true : false;
+		ie.has_v6only = true;
 	}
 
 	ie.src_addr = xmalloc(pb_repeated_size(&ie, src_addr));
@@ -359,6 +368,13 @@ static int open_inet_sk(struct file_desc *d)
 		return -1;
 	}
 
+	if (ii->ie->v6only) {
+		int yes = 1;
+
+		if (restore_opt(sk, SOL_IPV6, IPV6_V6ONLY, &yes) == -1)
+			return -1;
+	}
+
 	if (tcp_connection(ii->ie)) {
 		if (!opts.tcp_established_ok) {
 			pr_err("Connected TCP socket in image\n");