xfrm: fix __xfrm_route_forward()

This function should return 0 in case of error, 1 if OK
commit 452edd598f60522 (xfrm: Return dst directly from xfrm_lookup())
got it wrong.

Reported-and-bisected-by: Michael Smith <msmith@cbnco.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>