idr: make idr_get_next() good for rcu_read_lock()

Make one small adjustment to idr_get_next(): take the height from the
top layer (stable under RCU) instead of from the root (unprotected by
RCU), as idr_find() does: so that it can be used with RCU locking.
Copied comment on RCU locking from idr_find().

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>
Signed-off-by: garwedgess <garethwilliams21@gmail.com>