io_uring: use inflight_entry instead of compl.list

req->compl.list is used to cache freed requests, and so can't overlap in
time with req->inflight_entry. So, use inflight_entry to link requests
and remove compl.list.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e430e79d22d70a190d718831bda7bfed1daf8976.1628536684.git.asml.silence@gmail.com


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