rcutorture: Suppress boottime bad-sequence warnings

In normal production, an excessively long wait on a grace period
(synchronize_rcu(), for example) at boottime is often just as bad
as at any other time.  In fact, given the desire for fast boot, any
sort of long wait at boot is a bad idea.  However, heavy rcutorture
testing on large hyperthreaded systems can generate such long waits
during boot as a matter of course.  This commit therefore causes
the rcupdate.rcu_cpu_stall_suppress_at_boot kernel boot parameter to
suppress reporting of bootime bad-sequence warning due to excessively
long grace-period waits.

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