mm, gup: add missing refcount overflow checks on s390

mm, gup: add missing refcount overflow checks on s390

The mainline commit 8fde12ca79af ("mm: prevent get_user_pages() from
overflowing page refcount") was backported to 4.14.y stable as commit
04198de24771. The backport however missed that in 4.14, there are several
arch-specific gup.c versions with fast gup implementations, so these do not
prevent refcount overflow.

This stable-only commit fixes the s390 version, and is based on the backport in
SUSE SLES/openSUSE 4.12-based kernels.

The remaining architectures with own gup.c are sparc, mips, sh. It's unlikely
the known overflow scenario based on FUSE, which needs 140GB of RAM, is a
problem for those architectures, and I don't feel confident enough to patch
them.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>