powerpc/oprofile: Handle events that raise an exception without overflowing

powerpc/oprofile: Handle events that raise an exception without overflowing

Commit 0837e3242c73566fc1c0196b4ec61779c25ffc93 fixes a situation on POWER7
where events can roll back if a specualtive event doesn't actually complete.
This can raise a performance monitor exception.  We need to catch this to ensure
that we reset the PMC.  In all cases the PMC will be less than 256 cycles from
overflow.

This patch lifts Anton's fix for the problem in perf and applies it to oprofile
as well.

Signed-off-by: Eric B Munson <emunson@mgebm.net>
Cc: <stable@kernel.org> # as far back as it applies cleanly
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>