avoid calling zio_interrupt() from geom_up thread context. It turns out that
when provider is forcibly removed from the system and we kill worker thread
there can still be some ZIOs pending. To complete pending ZIOs when there is
no worker thread anymore we still have to call zio_interrupt() from geom_up
context. To avoid this race just remove use of worker threads altogether.
This should be more or less fine, because I also thought that zio_interrupt()
does more work, but it only makes small UMA allocation with M_WAITOK.
It also saves one context switch per I/O request.
PR: kern/145339
Reported by: Alex Bakhtin <Alex.Bakhtin@gmail.com>
MFC after: 1 week
4f54652124