io_uring/rw: preserve partial result for iopoll

io_uring/rw: preserve partial result for iopoll

A partial read will store the completed byte count in io->bytes_done.
The regular completion path applies io_fixup_rw_res() so that, when the
following operation reaches EOF, the number of bytes already read is
returned.

The iopoll completion path does not apply this fixup to the return value
and can return zero instead.

Use the fixup result when updating the CQE, and the raw result for the
reissue check.

Cc: stable@vger.kernel.org
Fixes: 4d9cb92ca41d ("io_uring/rw: fix short rw error handling")
Signed-off-by: Michael Wigham <michael@wigham.net>
Link: https://patch.msgid.link/20260613225240.34032-1-michael@wigham.net
Signed-off-by: Jens Axboe <axboe@kernel.dk>