io_u_qiter: Fix buffer overrun

io_u_qiter: Fix buffer overrun

In io_u_queue.h the io_u_qiter macro is loops around io_u_queue
structures. The problem comes with the end of loop initialisation:
i++, io_u = (q)->io_us[i]
For example, if io_us consists of one element and i is 0 then after the
first iteration is completed i++, io_u = (q)->io_us[i] will access
beyond the end of io_us.

Fix this by moving io_u initialisation to the expression part of the for
loop (yuck).

Found by Dr Memory.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Jens Axboe <axboe@fb.com>