CVE request - kernel: perf, powerpc: Handle events that raise an exception without overflowing

[Eugene Teo <eugene () redhat com>]

This does not affect the upstream kernel as it is already fixed. This is
an issue that was introduced when fixing https://bugzilla.redhat.com/740465.

A PMC is 32 bits (ie an int). When we pass it around as an unsigned
long, we need to cast it before doing the comparison. Using perf on
power machine with a local, unprivileged user account can cause a denial
of service.

Upstream commit:
http://git.kernel.org/linus/0837e3242c73566fc1c0196b4ec61779c25ffc93

Reference:
https://bugzilla.redhat.com/767914

Thanks, Eugene