ipvlan: fix incorrect usage of IS_ERR() macro in IPv6 code path.

ipvlan: fix incorrect usage of IS_ERR() macro in IPv6 code path.

The ip6_route_output() always returns a valid dst pointer unlike in IPv4
case. So the validation has to be different from the IPv4 path. Correcting
that error in this patch.

This was picked up by a static checker with a following warning -

   drivers/net/ipvlan/ipvlan_core.c:380 ipvlan_process_v6_outbound()
        warn: 'dst' isn't an ERR_PTR

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>