x86/mm/pae: Make pmd_t similar to pte_t

Instead of mucking about with at least 2 different ways of fudging
it, do the same thing we do for pte_t.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221022114424.580310787%40infradead.org