net/dst: Utility functions to build dst_metadata without supplying an skb

Extract __ip_tun_set_dst() and __ipv6_tun_set_dst() out of
ip_tun_rx_dst() and ipv6_tun_rx_dst(), to be used without supplying an
skb.

Signed-off-by: Amir Vadai <amir@vadai.me>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>