Modify send_pkt() to return the generated packet and have the caller
do the subsequent ip_output() in IPFW. In ipfw_tick(), the keep-alive
packets must be generated from the data that resides under the
stateful lock, but they must not be sent at that time, as this would
cause a lock order reversal with the normal ordering (interface's
lock, then locks belonging to the pfil hooks).

In practice, this caused deadlocks when using IPFW and if_bridge(4)
together to do stateful transparent filtering.

MFC after: 1 week