This is a simple hack for RELENG_7 (ie, it needs to be ported to -head first
before committed) which implements a log GEOM module. It is based off the nop
GEOM module.

The aim of the log module is to provide a method of logging bio queues
and completions for later analysis. At the moment the logging is simply done
via kernel printf()'s which will cause all manner of issues under any
reasonable load. The eventual aim is to have a lightweight-ish ring buffer
device, interfaced into kqueue for notification, to gather the disk profiling
information. I also hope to add queue/dequeue timestamping of some sort.

Adrian