ublk: improve detection and handling of ublk server exit

ublk: improve detection and handling of ublk server exit

There are currently two ways in which ublk server exit is detected by
ublk_drv:

1. uring_cmd cancellation. If there are any outstanding uring_cmds which
   have not been completed to the ublk server when it exits, io_uring
   calls the uring_cmd callback with a special cancellation flag as the
   issuing task is exiting.
2. I/O timeout. This is needed in addition to the above to handle the
   "saturated queue" case, when all I/Os for a given queue are in the
   ublk server, and therefore there are no outstanding uring_cmds to
   cancel when the ublk server exits.

There are a couple of issues with this approach:

- It is complex and inelegant to have two methods to detect the same
  condition
- The second method detects ublk server exit only after a long delay
  (~30s, the default timeout assigned by the block layer). This delays
  the nosrv behavior from kicking in and potential subsequent recovery
  of the device.

The second issue is brought to light with the new test_generic_06 which
will be added in following patch. It fails before this fix:

selftests: ublk: test_generic_06.sh
dev id is 0
dd: error writing '/dev/ublkb0': Input/output error
1+0 records in
0+0 records out
0 bytes copied, 30.0611 s, 0.0 kB/s
DEAD
dd took 31 seconds to exit (>= 5s tolerance)!
generic_06 : [FAIL]

Fix this by instead detecting and handling ublk server exit in the
character file release callback. This has several advantages:

- This one place can handle both saturated and unsaturated queues. Thus,
  it replaces both preexisting methods of detecting ublk server exit.
- It runs quickly on ublk server exit - there is no 30s delay.
- It starts the process of removing task references in ublk_drv. This is
  needed if we want to relax restrictions in the driver like letting
  only one thread serve each queue

There is also the disadvantage that the character file release callback
can also be triggered by intentional close of the file, which is a
significant behavior change. Preexisting ublk servers (libublksrv) are
dependent on the ability to open/close the file multiple times. To
address this, only transition to a nosrv state if the file is released
while the ublk device is live. This allows for programs to open/close
the file multiple times during setup. It is still a behavior change if a
ublk server decides to close/reopen the file while the device is LIVE
(i.e. while it is responsible for serving I/O), but that would be highly
unusual. This behavior is in line with what is done by FUSE, which is
very similar to ublk in that a userspace daemon is providing services
traditionally provided by the kernel.

With this change in, the new test (and all other selftests, and all
ublksrv tests) pass:

selftests: ublk: test_generic_06.sh
dev id is 0
dd: error writing '/dev/ublkb0': Input/output error
1+0 records in
0+0 records out
0 bytes copied, 0.0376731 s, 0.0 kB/s
DEAD
generic_04 : [PASS]

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250416035444.99569-6-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>