ublk: prepare for supporting to register request buffer automatically

ublk: prepare for supporting to register request buffer automatically

UBLK_F_SUPPORT_ZERO_COPY requires ublk server to issue explicit buffer
register/unregister uring_cmd for each IO, this way is not only inefficient,
but also introduce dependency between buffer consumer and buffer register/
unregister uring_cmd, please see tools/testing/selftests/ublk/stripe.c
in which backing file IO has to be issued one by one by IOSQE_IO_LINK.

Prepare for adding feature UBLK_F_AUTO_BUF_REG for addressing the existing
zero copy limitation:

- register request buffer automatically to ublk uring_cmd's io_uring
  context before delivering io command to ublk server

- unregister request buffer automatically from the ublk uring_cmd's
  io_uring context when completing the request

- io_uring will unregister the buffer automatically when uring is
  exiting, so we needn't worry about accident exit

For using this feature, ublk server has to create one sparse buffer table

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250520045455.515691-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>