mm: zswap: fix shrinker NULL crash with cgroup_disable=memory

mm: zswap: fix shrinker NULL crash with cgroup_disable=memory

Christian reports a NULL deref in zswap that he bisected down to the zswap
shrinker.  The issue also cropped up in the bug trackers of libguestfs [1]
and the Red Hat bugzilla [2].

The problem is that when memcg is disabled with the boot time flag, the
zswap shrinker might get called with sc->memcg == NULL.  This is okay in
many places, like the lruvec operations.  But it crashes in
memcg_page_state() - which is only used due to the non-node accounting of
cgroup's the zswap memory to begin with.

Nhat spotted that the memcg can be NULL in the memcg-disabled case, and I
was then able to reproduce the crash locally as well.

[1] https://github.com/libguestfs/libguestfs/issues/139
[2] https://bugzilla.redhat.com/show_bug.cgi?id=2275252

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
Signed-off-by: Johannes Weiner <[email protected]>
Reported-by: Christian Heusel <[email protected]>
Debugged-by: Nhat Pham <[email protected]>
Suggested-by: Nhat Pham <[email protected]>
Tested-by: Christian Heusel <[email protected]>
Acked-by: Yosry Ahmed <[email protected]>
Cc: Chengming Zhou <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Richard W.M. Jones <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Vitaly Wool <[email protected]>
Cc: <[email protected]>	[v6.8]
Signed-off-by: Andrew Morton <[email protected]>