x86, microcode, AMD: Simplify ucode verification

Basically, what we did until now is take out a chunk of the firmware
image, vmalloc space for it and inspect it before application. And
repeat.

This patch changes all that so that we look at each ucode patch from
the firmware image, check it for sanity and copy it to local buffer for
application only once and if it passes all checks. Thus, vmalloc-ing for
each piece is gone, we can do proper size checking only of the patch
which is destined for the CPU of the current machine instead of each
single patch, which is clearly wrong.

Oh yeah, simplify and cleanup the code while at it, along with adding
comments as to what actually happens.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>