rcu: Start timing stall repetitions after warning complete

Systems with low-bandwidth consoles can have very large printk()
latencies, and on such systems it makes no sense to have the next RCU CPU
stall warning message start output before the prior message completed.
This commit therefore sets the time of the next stall only after the
prints have completed.  While printing, the time of the next stall
message is set to ULONG_MAX/2 jiffies into the future.

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>