Currently, given a multi-level srcu_node tree, SRCU can scan the full
set of srcu_data structures at each level when cleaning up after a grace
period.  This, though harmless otherwise, represents pointless overhead.
This commit therefore eliminates this overhead by scanning the srcu_data
structures only when traversing the leaf srcu_node structures.
Signed-off-by: NIldar Ismagilov <devix84@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>