io_uring: remove drain prealloc checks

Currently io_drain_req() has two steps. The first is fast path checking
sequence numbers. The second is allocations, rechecking and actual
queuing. Further simplify it by removing the first step.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/4d06e89ed07611993d7bf89182de2300858379bd.1746788718.git.asml.silence@gmail.com


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