fix: io_uring sqpoll issue_time empty when kernel not yet read sq

In io_uring sqpoll mode, when kernel side thread has not yet read 
the sqring before second fio_ioring_commit() called, the 
sq_ring.head will remain the same. The second 
fio_ioring_commit() will initialize the wrong io_u's issue_time. 
The old(in head) io_u‘s issue_time will to be initialized twice and 
new(in tail - 1) io_u's issue_time will not to be initialized.
This problem will cause clat is weird, sometimes larger than lat.

Signed-off-by: Jueji Yang <jueji.yang@gmail.com>

engines/io_uring.c

@@ -646,7 +646,7 @@ static int fio_ioring_commit(struct thread_data *td)
  */
  if (o->sqpoll_thread) {
  struct io_sq_ring *ring = &ld->sq_ring;
- unsigned start = *ld->sq_ring.head;
+ unsigned start = *ld->sq_ring.tail - ld->queued;
  unsigned flags;
 
  flags = atomic_load_acquire(ring->flags);