ublk: use struct_size() for allocation

ublk: use struct_size() for allocation

Convert ublk_queue to use struct_size() for allocation.

Changes in this commit:

1. Update ublk_init_queue() to use struct_size(ubq, ios, depth)
   instead of manual size calculation (sizeof(struct ublk_queue) +
   depth * sizeof(struct ublk_io)).

This provides better type safety and makes the code more maintainable
by using standard kernel macro for flexible array handling.

Meantime annotate ublk_queue.ios by __counted_by().

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>