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

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>