memcg: let css_get_next() rely upon rcu_read_lock()

Remove lock and unlock around css_get_next()'s call to idr_get_next().
memcg iterators (only users of css_get_next) already did rcu_read_lock(),
and its comment demands that; but add a WARN_ON_ONCE to make sure of it.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: nobodyAtall <tsogkas@ceid.upatras.gr>
kmalloc: Finer grained memory allocation blocks for the ARM11 systems

(via ezterry)
(cherry picked from commit 23d06303c840fb0ecaac0122e1892a8fa26500f0)

Signed-off-by: nobodyAtall <tsogkas@ceid.upatras.gr>