cam: Avoiding waking up doneq threads if we're dumping

Depending on the state of the target doneq thread at the time of the
panic, the wakeup can hang indefinitely in thread_lock_block_wait().
That function should likely be modified to return immediately if the
scheduler is stopped, but it is also preferable to avoid wakeups in
general after a panic.

Reported by: pho
Reviewed by: mav, imp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32126