rtnetlink: do not depend on RTNL for IFLA_QDISC output

dev->qdisc can be read using RCU protection.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>