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

[markj]

Summary

Depending on the state of the doneq thread at the time of the panic, the
wakeup can hang indefinitely in thread_lock_block_wait(). I think that
function should be modified to return immediately if the scheduler is
stopped, but it also seems more robust to avoid wakeups in general after
a panic.

Reported by: pho

Test Plan

Peter reported occasional hangs while dumping core after a panic,
resolved by this patch.