io_uring/rw: always clear ->bytes_done on io_async_rw setup

A previous commit mistakenly moved the clearing of the in-progress byte
count into the section that's dependent on having a cached iovec or not,
but it should be cleared for any IO. If not, then extra bytes may be
added at IO completion time, causing potentially weird behavior like
over-reporting the amount of IO done.

Fixes: d7f11616 ("io_uring/rw: Allocate async data through helper")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202412271132.a09c3500-lkp@intel.com


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