ipv4: sendto/hdrincl: don't use destination address found in header

ipv4: raw_sendmsg: don't use header's destination address

A sendto() regression was bisected and found to start with commit
f8126f1d5136be1 (ipv4: Adjust semantics of rt->rt_gateway.)

The problem is that it tries to ARP-lookup the constructed packet's
destination address rather than the explicitly provided address.

Fix this using FLOWI_FLAG_KNOWN_NH so that given nexthop is used.

cf. commit 2ad5b9e4bd314fc685086b99e90e5de3bc59e26b

Reported-by: Chris Clark <chris.clark@alcatel-lucent.com>
Bisected-by: Chris Clark <chris.clark@alcatel-lucent.com>
Tested-by: Chris Clark <chris.clark@alcatel-lucent.com>
Suggested-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Chris Clark <chris.clark@alcatel-lucent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/raw.c

@@ -571,7 +571,8 @@ static int raw_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 	flowi4_init_output(&fl4, ipc.oif, sk->sk_mark, tos,
 			   RT_SCOPE_UNIVERSE,
 			   inet->hdrincl ? IPPROTO_RAW : sk->sk_protocol,
-			   inet_sk_flowi_flags(sk) | FLOWI_FLAG_CAN_SLEEP,
+			   inet_sk_flowi_flags(sk) | FLOWI_FLAG_CAN_SLEEP |
+			    (inet->hdrincl ? FLOWI_FLAG_KNOWN_NH : 0),
 			   daddr, saddr, 0, 0);
 
 	if (!inet->hdrincl) {