memcg: fix memcg_kmem_bypass() for remote memcg charging

memcg: fix memcg_kmem_bypass() for remote memcg charging

While trying to use remote memcg charging in an out-of-tree kernel
module I found it's not working, because the current thread is a
workqueue thread.

As we will probably encounter this issue in the future as the users of
memalloc_use_memcg() grow, and it's nothing wrong for this usage, it's
better we fix it now.

Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Link: http://lkml.kernel.org/r/1d202a12-26fe-0012-ea14-f025ddcd044a@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>