cam: clear stack-allocated CCB in the target layer

Note that, as pointed out by scottl@, this code should really look
a bit different, in that the stack allocations should be replaced
with dynamic allocation, and the periph creation should be moved
to a context where one can use M_WAITOK. See the review for more
details. For now let's go with a minimal fix until we're done with
UMA CCBs.

Reviewed By: mav, imp
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D30298