srcu: Improve rcu_seq grace-period-counter abstraction

The expedited grace-period code contains several open-coded shifts
know the format of an rcu_seq grace-period counter, which is not
particularly good style.  This commit therefore creates a new
rcu_seq_ctr() function that extracts the counter portion of the
counter, and an rcu_seq_state() function that extracts the low-order
state bit.  This commit prepares for SRCU callback parallelization,
which will require two state bits.

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