mlxsw: spectrum_router: Expose router struct to internal users

The dpipe code accesses internal router data structures and acquires
RTNL to protect against their changes. Subsequent patches will remove
reliance on RTNL and introduce a dedicated lock to protect router data
structures.

Publish the router struct to internal users such as the dpipe, so that
they could acquire it instead of RTNL.

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