mlxsw: spectrum_router: Do not assume RTNL is taken during nexthop init

RTNL is going to be removed from route insertion path, so use
__in_dev_get_rcu() from an RCU read-side critical section instead of
__in_dev_get_rtnl() which assumes RTNL is taken.

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