mlxsw: spectrum_router: Allocate discard adjacency entry when needed

Commit 0c3cbbf9 ("mlxsw: Add specific trap for packets routed via
invalid nexthops") allocated an adjacency entry during driver
initialization whose purpose is to discard packets hitting the route
pointing to it.

These adjacency entries are allocated from a resource called KVD linear
(KVDL). There are situations in which the user can decide to set the
size of this resource (via devlink-resource) to 0, in which case the
driver will not be able to load.

Therefore, instead of pre-allocating this adjacency entry, simply
allocate it only when needed. A variable indicating the validity of the
entry is added and is used to ensure it is only allocated and written
once and that it is freed after all the routes were flushed.

Fixes: 0c3cbbf9 ("mlxsw: Add specific trap for packets routed via invalid nexthops")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>