mlxsw: spectrum_router: Use different trap identifier for unresolved nexthops

Unresolved nexthops are currently written to the adjacency table with a
discard action. Packets hitting such entries are trapped to the CPU via
the 'DISCARD_ROUTER3' trap which can be enabled or disabled on demand,
but is always enabled in order to ensure the kernel can resolve the
unresolved neighbours.

This trap will be needed for blackhole nexthops support. Therefore, move
unresolved nexthops to explicitly program the adjacency entries with a
trap action and a different trap identifier, 'RTR_EGRESS0'.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>