ublk: convert to refcount_t

Convert to refcount_t and prepare for supporting to register bvec buffer
automatically, which needs to initialize reference counter as 2, and
kref doesn't provide this interface, so convert to refcount_t.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Suggested-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250520045455.515691-2-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>