x86/efi: Prevent mixed mode boot corruption with CONFIG_VMAP_STACK=y

x86/efi: Prevent mixed mode boot corruption with CONFIG_VMAP_STACK=y

Booting an EFI mixed mode kernel has been crashing since commit:

  ab6187a05c6e ("x86/mm/64: Enable vmapped stacks (CONFIG_HAVE_ARCH_VMAP_STACK=y)")

The user-visible effect in my test setup was the kernel being unable
to find the root file system ramdisk. This was likely caused by silent
memory or page table corruption.

Enabling CONFIG_DEBUG_VIRTUAL=y immediately flagged the thunking code as
abusing virt_to_phys() because it was passing addresses that were not
part of the kernel direct mapping.

Use the slow version instead, which correctly handles all memory
regions by performing a page table walk.

Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
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: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20161112210424.5157-3-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>