rcu: Stop stall warning from dumping stacks if grace period ends

Currently, once an RCU CPU stall warning decides to dump the stalling
CPUs' stacks, the rcu_dump_cpu_stacks() function persists until it
has gone through the full list.  Unfortunately, if the stalled grace
periods ends midway through, this function will be dumping stacks of
innocent-bystander CPUs that happen to be blocking not the old grace
period, but instead the new one.  This can cause serious confusion.

This commit therefore stops dumping stacks if and when the stalled grace
period ends.

[ paulmck: Apply Joel Fernandes feedback. ]

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