iptunnel: specify protocol outside IP header

Before this patch, ip_tunnel_xmit() was using the field protocol from the IP
header passed into argument.
There is no functional change, this patch prepares the support of IPv4 over
IPv4 for module sit.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>