rcu-tasks: Be more patient for RCU Tasks boot-time testing

The RCU-Tasks family of grace-period primitives can take some time to
complete, and the amount of time can depend on the exact hardware and
software configuration.  Some configurations boot up fast enough that the
RCU-Tasks verification process gets false-positive failures.  This commit
therefore allows up to 30 seconds for the grace periods to complete, with
this value adjustable downwards using the rcupdate.rcu_task_stall_timeout
kernel boot parameter.

Reported-by: Matthew Wilcox <willy@infradead.org>
Reported-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>