x86/microcode/AMD: Concentrate patch verification

Add a verify_patch() function which tries to sanity-check many aspects
of a microcode patch supplied by an outside container before attempting
a load.

Prepend all sub-functions' names which verify an aspect of a microcode
patch with "__".

Call it in verify_and_add_patch() *before* looking at the microcode
header.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20181107170218.7596-7-bp@alien8.de