rcu-tasks: Convert rcu_tasks_wait_gp() for-loop to while-loop

rcu-tasks: Convert rcu_tasks_wait_gp() for-loop to while-loop

The infinite for-loop in rcu_tasks_wait_gp() has its only exit at the
top of the loop, so this commit does the straightforward conversion to
a while-loop, thus saving a few lines.

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