mm: introduce mm_[p4d|pud|pmd]_folded

[ Upstream commit 1071fc5779d9846fec56a4ff6089ab08cac1ab72 ]

Add three architecture overrideable functions to test if the
p4d, pud, or pmd layer of a page table is folded or not.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>