Crash recovery for user-space block drivers

AFAIK, the above seems not true.

IMO, there are lots of jobs which can be done by ublk, such as some generic things:

- re-implement nbd-client with ublk and io_uring, especially applying recent io_uring/net zero copy feature
- ublk export for qemu-storage-daemon
- SPDK backend for UBD mod
- ceph's rbd
- ublk based zoned target
- ublk base compression target
...

Some of them have been in-progress. Userspace development is easier and more efficient, but still takes
a while, especially just 4 months past since the first version of ublk driver(RFC) is posted.