[PATCH STABLE ONLY] add missing page refcount overflow checks

This collection of patches add the missing overflow checks in arch-specific gup.c variants for x86 and s390. Those were missed in backport of 8fde12ca79af ("mm: prevent get_user_pages() from overflowing page refcount") as mainline had a single gup.c implementation at that point. See individual patches for details.

Vlastimil