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

A sendto() regression was bisected and found to start with commit
f8126f1d (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 2ad5b9e4Reported-by: NChris Clark <chris.clark@alcatel-lucent.com>
Bisected-by: NChris Clark <chris.clark@alcatel-lucent.com>
Tested-by: NChris Clark <chris.clark@alcatel-lucent.com>
Suggested-by: NJulian Anastasov <ja@ssi.bg>
Signed-off-by: NChris Clark <chris.clark@alcatel-lucent.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>