[PATCH] x86-64: adjust pmd_bad()

Make pmd_bad() symmetrical to pgd_bad() and pud_bad(). At once,
simplify them all.

TBD: tighten down the checks again as suggested by Hugh D.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>