xfrm: add route lookup to xfrm4_rcv_encap

At this point, with TCP encapsulation, the dst may be gone, but
xfrm_input needs one.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>