rcu-tasks: Move RTGS_WAIT_CBS to beginning of rcu_tasks_kthread() loop

Early in debugging, it made some sense to differentiate the first
iteration from subsequent iterations, but now this just causes confusion.
This commit therefore moves the "set_tasks_gp_state(rtp, RTGS_WAIT_CBS)"
statement to the beginning of the "for" loop in rcu_tasks_kthread().

Reported-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>