memcg: avoid vmpressure oops when memcg disabled

A CONFIG_MEMCG=y kernel booted with "cgroup_disable=memory" crashes on a
NULL memcg (but non-NULL root_mem_cgroup) when vmpressure kicks in.
Here's the patch I use to avoid that, but you might prefer a test on
mem_cgroup_disabled() somewhere.

Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/vmpressure.c

@@ -260,7 +260,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
 		enum vmpressure_levels level;
 
 		/* For now, no users for root-level efficiency */
-		if (memcg == root_mem_cgroup)
+		if (!memcg || memcg == root_mem_cgroup)
 			return;
 
 		spin_lock(&vmpr->sr_lock);