x86, microcode: Don't request microcode from userspace unnecessarily

Requesting the microcode from userspace *every time* when onlining CPUs
(during a CPU hotplug operation) is unnecessary. Thus, ensure that
once the kernel gets the microcode after booting, it is not freed nor
invalidated when a CPU goes offline, so that it can be reused when that
CPU comes back online, without requesting userspace for it again. As a
result, the CPU hotplug operations become faster as well.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Link: http://lkml.kernel.org/r/4E91F908.5010006@linux.vnet.ibm.com
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>