siftr: refactor batch log processing

[rscheff]

Summary

D41175 introduced a condition, where free could be called
while in a critical section or under spinlock.

Refactoring the code to perform the batch processing of
log messaged in two phases, first cycling through a limited
number of collected packets, and only afterwards freeing
these processed packets.

Test Plan

Repro was done using a VM with packetdrill -v on a
script also starting a tcpdump, with near 100% reproducability.

These changes fix the panic, while retaining the batch
log processing.

[rscheff]

• replace magic number with defined variable

[cc]

I think I have tested this method of separating packet processing and free before. But I want to make sure it is performing as expected. I will give my comment after testing.

[cc]

sys/netinet/siftr.c

521

Add a space between ";cnt > 0" for readability.

[cc]

Tested in Emulab.net. Looks good to me.