io_uring: fix retry handling off iowq

io_uring: fix retry handling off iowq

io_req_complete_post() doesn't handle reissue and if called with a
REQ_F_REISSUE request it might post extra unexpected completions. Fix it
by pushing into flush_completion via task work.

Fixes: d803d123948fe ("io_uring/rw: handle -EAGAIN retry at IO completion time")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/badb3d7e462881e7edbfcc2be6301090b07dbe53.1742829388.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>