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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ed8ef7ae8b05d9f73db2182b5e24b0b76b0783a2)