x86/microcode: Fix loading precedence

So it can happen that even with builtin microcode,
CONFIG_BLK_DEV_INITRD=y gets forgotten enabled.

Or, even with that disabled, an initrd image gets supplied by the boot
loader, by omission or is simply forgotten there. And since we do look
at boot_params.hdr.ramdisk_* to know whether we have received an initrd,
we might get puzzled.

So let's just make the loader look for builtin microcode first and if
found, ignore the ramdisk image.

If no builtin found, it falls back to scanning the supplied initrd, of
course.

For that, we move all the initrd scanning in a separate
__scan_microcode_initrd() function and fall back to it only if
load_builtin_intel_microcode() has failed.

Reported-and-tested-by: Gabriel Craciunescu <nix.or.die@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1465225850-7352-2-git-send-email-bp@alien8.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>