thinktime: Avoid calculating a negative time left to wait

thinktime: Avoid calculating a negative time left to wait

When the thinktime_spin option specifies a value that is within
a few milliseconds of the thinktime value, in handle_thinktime()
it's possible in a VM environment for the duration of usec_spin()
to exceed the thinktime value. While doing usec_spin(), the vCPU
could get de-scheduled or the hypervisor could steal CPU time
from the vCPU. When the guest vCPU runs after being scheduled
again, it may read the clock and find that more time has elapsed
than intended. In such a case, the time left to wait could be
calculated as a negative value. Subsequent calculations then go
awry because the time left is cast as unsigned.

Fix this by detecting when the time left would go negative and
just set it to zero.

Fixes: 1a9bf8146 ("Add option to ignore thinktime for rated IO")
Fixes: https://github.com/axboe/fio/issues/1588
Link: https://lore.kernel.org/fio/1689354334-131024-1-git-send-email-mikelley@microsoft.com/T/#u
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>