x86/mm: Simplify PAE PGD sharing macros

x86/mm: Simplify PAE PGD sharing macros

There are a few too many levels of abstraction here.

First, just expand the PREALLOCATED_PMDS macro in place to make it
clear that it is only conditional on PTI.

Second, MAX_PREALLOCATED_PMDS is only used in one spot for an
on-stack allocation. It has a *maximum* value of 4. Do not bother
with the macro MAX() magic.  Just set it to 4.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20250414173238.6E3CDA56%40davehans-spike.ostc.intel.com