rcu: Remove last non-flavor-traversal rsp local variable from tree_plugin.h

This commit removes the last non-flavor-traversal rsp local variable from
kernel/rcu/tree_plugin.h in favor of &rcu_state.  The flavor-traversal
locals will be removed with the removal of flavor traversal.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>