s390/mm: make pmd/pud_deref() large page aware

pmd/pud_deref() assume that they will never operate on large pmd/pud
entries, and therefore only use the non-large _xxx_ENTRY_ORIGIN mask.
With commit 9ec8fa8d ("s390/vmemmap: extend modify_pagetable()
to handle vmemmap"), that assumption is no longer true, at least for
pmd_deref().

In theory, we could end up with wrong addresses because some of the
non-address bits of a large entry would not be masked out.
In practice, this does not (yet) show any impact, because vmemmap_free()
is currently never used for s390.

Fix pmd/pud_deref() to check for the entry type and use the
_xxx_ENTRY_ORIGIN_LARGE mask for large entries.

While at it, also move pmd/pud_pfn() around, in order to avoid code
duplication, because they do the same thing.

Fixes: 9ec8fa8d ("s390/vmemmap: extend modify_pagetable() to handle vmemmap")
Cc: <stable@vger.kernel.org> # 5.9
Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>