inet: lockless getsockopt(IP_MTU)

sk_dst_get() does not require socket lock.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_sockglue.c

@@ -1620,13 +1620,6 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 			return -EFAULT;
 		return 0;
 	}
-	}
-
-	if (needs_rtnl)
-		rtnl_lock();
-	sockopt_lock_sock(sk);
-
-	switch (optname) {
 	case IP_MTU:
 	{
 		struct dst_entry *dst;
@@ -1636,12 +1629,17 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 			val = dst_mtu(dst);
 			dst_release(dst);
 		}
-		if (!val) {
-			sockopt_release_sock(sk);
+		if (!val)
 			return -ENOTCONN;
+		goto copyval;
 	}
-		break;
 	}
+
+	if (needs_rtnl)
+		rtnl_lock();
+	sockopt_lock_sock(sk);
+
+	switch (optname) {
 	case IP_UNICAST_IF:
 		val = (__force int)htonl((__u32) inet->uc_index);
 		break;