rtnetlink: do not depend on RTNL for many attributes

Following device fields can be read locklessly
in rtnl_fill_ifinfo() :

type, ifindex, operstate, link_mode, mtu, min_mtu, max_mtu, group,
promiscuity, allmulti, num_tx_queues, gso_max_segs, gso_max_size,
gro_max_size, gso_ipv4_max_size, gro_ipv4_max_size, tso_max_size,
tso_max_segs, num_rx_queues.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>