rcu: Make synchronize_rcu_expedited() fast path update .expedited_sequence

rcu: Make synchronize_rcu_expedited() fast path update .expedited_sequence

This commit causes the early boot single-CPU synchronize_rcu_expedited()
fastpath to update the rcu_state structure's ->expedited_sequence
counter.  This will allow the full-state polled grace-period APIs to
detect all expedited grace periods without the need to track the special
combined polling-only counter, which is another step towards removing
the ->rgos_polled field from the rcu_gp_oldstate, thereby reducing its
size by one third.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>