block size is one very key setting for block layer, and bad block size
could panic kernel easily.

Make sure that block size is set correctly.

Meantime if ublk_validate_params() fails, clear ub->params so that disk
is prevented from being added.

Fixes: 71f28f31 ("ublk_drv: add io_uring based userspace block driver")
Reported-and-tested-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>