Alter the order of checking time exceeded vs getting an io_u

Alter the order of checking time exceeded vs getting an io_u

I was hitting a case where fio would loop for ever even though the
runtime was exceeded if it had difficulty getting the io_u struct, so
I suggest this change to the code so it wld not loop forever.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>