tunnel: introduce udp_tun_rx_dst()

Introduce function udp_tun_rx_dst() to initialize tunnel dst on
receive path.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Reviewed-by: Jesse Gross <jesse@nicira.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>