um: time-travel: fix time corruption

In the Linux kernel, the following vulnerability has been resolved:

um: time-travel: fix time corruption

In 'basic' time-travel mode (without =inf-cpu or =ext), we

still get timer interrupts. These can happen at arbitrary

points in time, i.e. while in timer_read(), which pushes

time forward just a little bit. Then, if we happen to get

the interrupt after calculating the new time to push to,

but before actually finishing that, the interrupt will set

the time to a value that's incompatible with the forward,

and we'll crash because time goes backwards when we do the

forwarding.

Fix this by reading the time_travel_time, calculating the

adjustment, and doing the adjustment all with interrupts

disabled.