x86/mm/pat: Convert populate_pgd() to use page table apis

x86/mm/pat: Convert populate_pgd() to use page table apis

Use the ptdesc APIs for all page table allocation and free sites to allow
their separate allocation from struct page in the future. Convert the
remaining get_zeroed_page() calls to the generic page table APIs, as they
already use ptdescs.

Pass through init_mm since these are kernel page tables, as
both functions require it to identify kernel page tables. Because the
generic implementations do not use the second argument, pass a
placeholder to avoid reimplementing them or risking breakage on other
architectures.

It is not obvious whether these pages are freed. Regardless, convert the
remaining free paths as needed, noting that the only other possible free
paths have already been converted and that a frozen page table test
kernel has not reported any issues.

Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Link: https://patch.msgid.link/20260303194828.1406905-4-vishal.moola@gmail.com