Normally, when a gif(4) interface is a member of bridge(4),
bridge_enqueue() only calls gif_transmit(), so this code path should
never be reached. However, if the outer protocol of the gif tunnel is
IPv6, certain special IPv6 multicast control messages, such as MLDv2,
destinated for ff02::/16 may still reach gif_output() due to internal
behavior of IPv6 stack.
Therefore, the kernel should not panic because of such packets; instead,
they should be silently dropped.
PR: 227450
Fixes: 8a030872 ("gif(4): Assert that gif_output() isn't called for EtherIP")