mlxsw: spectrum_router: Trap packets hitting anycast routes

mlxsw: spectrum_router: Trap packets hitting anycast routes

I relied on the fact that anycast routes use the loopback device as
their nexthop device to trap packets hitting them to the CPU.

After commit 1ad23e7d336a ("net: ipv6: put host and anycast routes on
device with address") this is no longer the case and such routes are
programmed with a forward action (note the 'offload' flag):

anycast cafe:: dev enp3s0np7 proto kernel metric 0 offload pref medium

This will prevent the router from locally receiving packets destined to
the Subnet-Router anycast address.

Fix this by specifically programming anycast routes with action trap,
which results in the following output:

anycast cafe:: dev enp3s0np7 proto kernel metric 0 pref medium

Fixes: 1ad23e7d336a ("net: ipv6: put host and anycast routes on device with address")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>