x86/efi: Free efi_pgd with free_pages()

The efi_pgd is allocated as PGD_ALLOCATION_ORDER pages and therefore must
also be freed as PGD_ALLOCATION_ORDER pages with free_pages().

Fixes: d9e9a641

 ("x86/mm/pti: Allocate a separate user PGD")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1521746333-19593-1-git-send-email-longman@redhat.com