selftests: ublk: add single sqe allocator helper

Unify the sqe allocator helper, and we will use it for supporting
more cases, such as ublk stripe, in which variable sqe allocation
is required.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250322093218.431419-3-ming.lei@redhat.com


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