hugetlb_cgroup: fix illegal access to memory

This appears to be a mistake in commit faced7e0 ("mm: hugetlb
controller for cgroups v2").

Essentially that commit does a hugetlb_cgroup_from_counter assuming that
page_counter_try_charge has initialized counter.

But if that has failed then it seems will not initialize counter, so
hugetlb_cgroup_from_counter(counter) ends up pointing to random memory,
causing kasan to complain.

The solution is to simply use 'h_cg', instead of
hugetlb_cgroup_from_counter(counter), since that is a reference to the
hugetlb_cgroup anyway.  After this change kasan ceases to complain.

Fixes: faced7e0 ("mm: hugetlb controller for cgroups v2")
Reported-by: syzbot+cac0c4e204952cf449b1@syzkaller.appspotmail.com
Signed-off-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Giuseppe Scrivano <gscrivan@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: David Rientjes <rientjes@google.com>
Link: http://lkml.kernel.org/r/20200313223920.124230-1-almasrymina@google.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb_cgroup.c

@@ -240,8 +240,7 @@ int hugetlb_cgroup_charge_cgroup(int idx, unsigned long nr_pages,
 	if (!page_counter_try_charge(&h_cg->hugepage[idx], nr_pages,
 				     &counter)) {
 		ret = -ENOMEM;
-		hugetlb_event(hugetlb_cgroup_from_counter(counter, idx), idx,
-			      HUGETLB_MAX);
+		hugetlb_event(h_cg, idx, HUGETLB_MAX);
 	}
 	css_put(&h_cg->css);
 done: