the inter-value histogram for 2000 samples. If the width is 3 or less
for 10 consequtive samples, we trust the counter to be good, otherwise
we use the *_safe() method.
This method may be too strict, but the worst which can happen is that
we take the performance hit of the *_safe() method when we should not.
Make the *_safe() method more discriminating by mandating that the three
samples do not span more than 15 ticks on the counter.
Disable the PCI-ident based probing as a means to recognize good
counters.
Inspiration from: dillon and msmith
cb877d0050