io_uring: fix spurious drain flushing

io_uring: fix spurious drain flushing

io_queue_deferred() is not tolerant to spurious calls not completing
some requests. You can have an inflight drain-marked request and another
request that came after and got queued into the drain list. Now, if
io_queue_deferred() is called before the first request completes, it'll
check the 2nd req with req_need_defer(), find that there is no drain
flag set, and queue it for execution.

To make io_queue_deferred() work, it should at least check sequences for
the first request, and then we need also need to check if there is
another drain request creating another bubble.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/972bde11b7d4ef25b3f5e3fd34f80e4d2aa345b8.1746788718.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>