illumos gate

dumping to /dev/zvol/dsk/rpool/dump, offset 65536, content: kernel
dumping:  0:00   1% done
  1% done: 6146 pages dumped, dump failed: error 11

The problem was introduced in #8628, which works correctly under normal conditions, but when the system is panicking, semaphores behavior becomes inconsistent:

- sema_p() would allow the count to go down to 0, and NOT block
- sema_tryp() used in io submission queue would then never succeeded
- and sema_v() would NOT increase the count at all

So the operation seem to go normal, with one cmd at time during panic -- we get cmd to the queue, do a sema_tryp() (count goes down), cmd completed, we take it out of the queue, do a sema_v() (count does NOT go up), so we return EAGAIN after 'count' cmds have been processed.

The fix here is to make semaphores complete no-op during panic.