ip6_tunnel: simplify transmit path

Merge ip{4,6}ip6_tnl_xmit functions into one universal
ipxip6_tnl_xmit in preparation for adding MPLS support.

Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>