x86/efi: Clean up a minor mistake in comment

x86/efi: Clean up a minor mistake in comment

EFI allocates runtime services regions from EFI_VA_START, -4G, down
to -68G, EFI_VA_END - 64G altogether, top-down.

The mechanism was introduced in commit:

  d2f7cbe7b26a7 ("x86/efi: Runtime services virtual mapping")

Fix the comment that still says bottom-up.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20170404160245.27812-10-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>