rcu: Make rcu_seq_diff() more exact

The current implementatation of rcu_seq_diff() follows tradition in
providing a rough-and-ready approximation of the number of elapsed grace
periods between the two rcu_seq values.  However, this difference is
used to flag RCU-failure "near misses", which can be a valuable debugging
aid, so more exactitude would be an improvement.  This commit therefore
improves the accuracy of rcu_seq_diff().

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