powerpc/perf_event: Skip updating kernel counters if register value shrinks

Because of speculative event roll back, it is possible for some event coutners
to decrease between reads on POWER7.  This causes a problem with the way that
counters are updated.  Delta calues are calculated in a 64 bit value and the
top 32 bits are masked.  If the register value has decreased, this leaves us
with a very large positive value added to the kernel counters.  This patch
protects against this by skipping the update if the delta would be negative.
This can lead to a lack of precision in the coutner values, but from my testing
the value is typcially fewer than 10 samples at a time.

Signed-off-by: Eric B Munson <emunson@mgebm.net>
Cc: stable@kernel.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>