ipv6: Fix potential uninit-value access in __ip6_make_skb()

As it was done in commit fc1092f ("ipv4: Fix uninit-value access in
__ip_make_skb()") for IPv4, check FLOWI_FLAG_KNOWN_NH on fl6->flowi6_flags
instead of testing HDRINCL on the socket to avoid a race condition which
causes uninit-value access.

Fixes: ea30388 ("ipv6: Fix an uninit variable access bug in __ip6_make_skb()")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -1933,7 +1933,7 @@ struct sk_buff *__ip6_make_skb(struct sock *sk,
 		u8 icmp6_type;
 
 		if (sk->sk_socket->type == SOCK_RAW &&
-		   !inet_test_bit(HDRINCL, sk))
+		   !(fl6->flowi6_flags & FLOWI_FLAG_KNOWN_NH))
 			icmp6_type = fl6->fl6_icmp_type;
 		else
 			icmp6_type = icmp6_hdr(skb)->icmp6_type;