selftests: ublk: improve I/O ordering test with bpftrace

selftests: ublk: improve I/O ordering test with bpftrace

Remove test_generic_01.sh since block layer may reorder I/O, making
the test prone to false positives. Apply the improvements to
test_generic_02.sh instead, which supposes for covering ublk dispatch
io order.

Rework test_generic_02 to verify that ublk dispatch doesn't reorder I/O
by comparing request start order with completion order using bpftrace.

The bpftrace script now:
- Tracks each request's start sequence number in a map keyed by sector
- On completion, verifies the request's start order matches expected
  completion order
- Reports any out-of-order completions detected

The test script:
- Wait bpftrace BEGIN code block is run
- Pins fio to CPU 0 for deterministic behavior
- Uses block_io_start and block_rq_complete tracepoints
- Checks bpftrace output for reordering errors

Reported-and-tested-by: Alexander Atanasov <alex@zazolabs.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>