lwtunnel: ip tunnel: fix multiple routes with different encap

Currently, two routes going through the same tunnel interface are considered
the same even when they are routed to a different host after encapsulation.
This causes all routes added after the first one to have incorrect
encapsulation parameters.

This is nicely visible by doing:

  # ip r a 192.168.1.2/32 dev vxlan0 tunnel dst 10.0.0.2
  # ip r a 192.168.1.3/32 dev vxlan0 tunnel dst 10.0.0.3
  # ip r
  [...]
  192.168.1.2/32 tunnel id 0 src 0.0.0.0 dst 10.0.0.2 [...]
  192.168.1.3/32 tunnel id 0 src 0.0.0.0 dst 10.0.0.2 [...]

Implement the missing comparison function.

Fixes: 3093fbe7

 ("route: Per route IP tunnel metadata via lightweight tunnel")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>