siftr: refactor batch log processing

Refactoring to perform the batch processing of
log messaged in two phases. First cycling through a limited
number of collected packets, and only thereafter freeing
the processed packets. This prevents any chance of calling
free while in a critical / spinlocked section.

Reviewed By: tuexen
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D42949

(cherry picked from commit a95cd6e4870b79178860e03366c4327e533ecf1e)