mlxsw: spectrum_router: Do not assume RTNL is taken during RIF teardown

IPv6 addresses are deleted in an atomic context, so the driver defers
the potential teardown of the associated router interface (RIF) to a
work item that takes RTNL.

The RIF is only destroyed if the associated netdev does not have any IP
addresses (both IPv4 and IPv6). The IPv4 device ('struct in_device') is
currently fetched via __in_dev_get_rtnl() which assumes RTNL is taken.

Since RTNL is going to be removed, convert it to use __in_dev_get_rcu()
from an RCU read-side critical section.

Note that the IPv6 device ('struct inet6_dev') is fetched via
__in6_dev_get(), which does not require 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>