ipv6: Refactor common ip6gre_tunnel_init codes

It is a prep work to fix the dst_entry refcnt bugs in ip6_tunnel.

This patch refactors some common init codes used by both
ip6gre_tunnel_init and ip6gre_tap_init.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>