ioengines: Fix td->io_issues[ddir] over decrement

ioengines: Fix td->io_issues[ddir] over decrement

If an ioengine returned FIO_Q_BUSY, td->io_issues[ddir] would be
over decremented: once in td_io_queue() and then again in
requeue_io_u(). This was because td_io_queue() reverted various values
upon finding the queue busy but requeue_io_u() found the I/O was still
in flight so decremented again. This then showed up as unsigned
underflow on td->io_issues[ddir].

Fix this by clearing IO_U_F_FLIGHT on the I/O when we find the queue was
busy in td_io_queue().

Signed-off-by: Sitsofe Wheeer <sitsofe@yahoo.com>