Improve NVMe hot unplug handling.

Improve NVMe hot unplug handling.

If device is unplugged from the system (CSTS register reads return
0xffffffff), it makes no sense to send any more recovery requests or
expect any responses back.  If there is a detach call in such state,
just stop all activity and free resources.  If there is no detach
call (hot-plug is not supported), rely on normal timeout handling,
but when it trigger controller reset, do not wait for impossible and
quickly report failure.

MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.