x86/vdso: Fix gettimeofday masking

Because of how the virtual clocks use U64_MAX as an exception value
instead of a valid time, the clocks can no longer be assumed to wrap
cleanly. This is then compounded by arch_vdso_cycles_ok() rejecting
everything with the MSB/Sign-bit set.

Therefore, the effective mask becomes S64_MAX, and the comment with
vdso_calc_delta() that states the mask is U64_MAX and isn't optimized
out is just plain silly.

Now, the code has a negative filter -- to deal with TSC wobbles:

	if (cycles > last)

which is just plain wrong, because it should've been written as:

	if ((s64)(cycles - last) > 0)

to take wrapping into account, but per all the above, we don't
actually wrap on u64 anymore.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>  # Hyper-V
Link: https://lore.kernel.org/r/20230519102715.704767397@infradead.org