This commit makes rcu_try_advance_all_cbs() use ->gp_seq.  It uses
rcu_seq_ctr() in order to shift away the state bits, so that the
low-order bits of the result may safely be used to index ->nocb_gp_wq[].
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>