io_uring/rw: fix error'ed retry return values

Kernel test robot reports that we test negativity of an unsigned in
io_fixup_rw_res() after a recent change, which masks error codes and
messes up the return value in case I/O is re-retried and failed with
an error.

Fixes: 4d9cb92 ("io_uring/rw: fix short rw error handling")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9754a0970af1861e7865f9014f735c70dc60bf79.1663071587.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/rw.c

@@ -206,7 +206,7 @@ static bool __io_complete_rw_common(struct io_kiocb *req, long res)
 	return false;
 }
 
-static inline unsigned io_fixup_rw_res(struct io_kiocb *req, unsigned res)
+static inline int io_fixup_rw_res(struct io_kiocb *req, long res)
 {
 	struct io_async_rw *io = req->async_data;