ip_gre: make ipgre_tunnel_xmit() not parse network header as IP unconditionally

ipgre_tunnel_xmit() parses network header as IP unconditionally.
But transmitting packets are not always IP packet. For example such packet
can be sent by packet socket with sockaddr_ll.sll_protocol set.
So make the function check if skb->protocol is IP.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>