mlxsw: spectrum_router: Introduce router lock

Introduce a mutex to protect the internal structure of the routing code.
A single lock is added instead of a more fine-grained and complicated
locking scheme because there is not a lot of concurrency in the routing
code.

The main motivation is remove the dependence on RTNL lock, which is
currently used by both the process pushing routes to the kernel and the
workqueue pushing the routes to the underlying device.

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