exit_batch_rtnl() is called while RTNL is held,
and devices to be unregistered can be queued in the dev_kill_list.

This saves one rtnl_lock()/rtnl_unlock() pair,
and one unregister_netdevice_many() call.

Note: it should be possible to remove the synchronize_net()
call from geneve_sock_release() in a future patch.

v4: move WARN_ON_ONCE(!list_empty(&gn->sock_list))
   into geneve_exit_net(), after devices have been unregistered.
   (Antoine Tenart feedback)
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Antoine Tenart <atenart@kernel.org>
Link: https://lore.kernel.org/r/20240206144313.2050392-7-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>