mlxsw: spectrum_router: Take router lock from exported helpers

The routing code exports some helper functions that can be called from
other driver modules such as the bridge. These helpers are never called
with the router lock already held and therefore need to take it in order
to serialize access to shared router structures.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>