rcu: Make synchronize_sched_expedited() better at work sharing

When synchronize_sched_expedited() takes its second and subsequent
snapshots of sync_sched_expedited_started, it subtracts 1.  This
means that the concurrent caller of synchronize_sched_expedited()
that incremented to that value sees our successful completion, it
will not be able to take advantage of it.  This restriction is
pointless, given that our full expedited grace period would have
happened after the other guy started, and thus should be able to
serve as a proxy for the other guy successfully executing
try_stop_cpus().

This commit therefore removes the subtraction of 1.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>