Rework vtblk dump handling of in flight requests

Previously, the driver resets the device and abandon the requests that
are caught in flight when the dump was initiated. This was problematic
if the system is resumed after the dump is completed.

While that is probably not the typical action, it is simple to rework
the driver to very likely have the device usable after the dump without
making it more likely for the dump to fail. The in flight requests are
simply queued for completion once the dump is finished.

Requested by: markj
MFC after: 1 month