x86/mm: Don't free P4D table when it is folded at runtime

x86/mm: Don't free P4D table when it is folded at runtime

When the P4D page table layer is folded at runtime, the p4d_free()
should do nothing, the same as in <asm-generic/pgtable-nop4d.h>.

It seems this bug should cause double-free in efi_call_phys_epilog(),
but I don't know how to trigger that code path, so I can't confirm that
by testing.

Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org # 4.17
Fixes: 98219dda2ab5 ("x86/mm: Fold p4d page table layer at runtime")
Link: http://lkml.kernel.org/r/20180625102427.15015-1-aryabinin@virtuozzo.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>