thoughts on system hangs after panic()

I suspect the cause of the system hangs after a panic that are keeping
crash dumps from working is one of the shutdown_pre_sync handlers
getting wedged.  The most likely culprit is syncer_shutdown(), but I
think any of the handlers could get wedged in kthread_suspend().

Either we should skip shutdown_pre_sync entirely if RB_NOSYNC is set, or
the handlers should obey this bit as appropriate.  The latter would
require a little more code to be touched, but would allow for more
flexibility.