cam: make sure to clear CCBs allocated on the stack

This is required for small CCBs support, where we need to track
whether the CCB was allocated from an UMA zone or not. There are
no (intended) functional changes with the current source.

Reviewed By: imp
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D29484