ublk_drv: comment on ublk_driver entry of Kconfig

Add help info for choosing to build ublk_drv as module or builtin.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221029010432.598367-3-ming.lei@redhat.com


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