ip/options: explicitly provide net ns to __ip_options_echo()

__ip_options_echo() uses the current network namespace, and
currently retrives it via skb->dst->dev.

This commit adds an explicit 'net' argument to __ip_options_echo()
and update all the call sites to provide it, usually via a simpler
sock_net().

After this change, __ip_options_echo() no more needs to access
skb->dst and we can drop a couple of hack to preserve such
info in the rx path.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index b631ec6..73b0b15 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1525,7 +1525,7 @@
 	int err;
 	int oif;
 
-	if (__ip_options_echo(&replyopts.opt.opt, skb, sopt))
+	if (__ip_options_echo(net, &replyopts.opt.opt, skb, sopt))
 		return;
 
 	ipc.addr = daddr;