mlxsw: spectrum_router: Do not assume RTNL is taken when resolving underlay device

The function that resolves the underlay device of the IPIP tunnel
assumes that RTNL is taken, but this will not be correct when RTNL is
removed from the route insertion path.

Convert the function to use dev_get_by_index_rcu() instead of
__dev_get_by_index() and make sure it is always called from an RCU
read-side critical section.

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