mm: move paddr_to_pdx()

mm: move paddr_to_pdx()

There's nothing arch-specific about it.

While there, on x86 visually separate the vmap_to_*() macros from those
covered by the earlier comment.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Oleksii Kurochko<oleksii.kurochko@gmail.com>