io_uring: fix rw req completion

WARNING: at fs/io_uring.c:8578 io_ring_exit_work.cold+0x0/0x18

As reissuing is now passed back by REQ_F_REISSUE and kiocb_done()
internally uses __io_complete_rw(), it may stop after setting the flag
so leaving a dangling request.

There are tricky edge cases, e.g. reading beyound file, boundary, so
the easiest way is to hand code reissue in kiocb_done() as
__io_complete_rw() was doing for us before.

Fixes: 230d50d4

 ("io_uring: move reissue into regular IO path")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f602250d292f8a84cca9a01d747744d1e797be26.1617842918.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>