rcu: Forgive slow expedited grace periods at boot time

Boot-time processing often loops in the kernel longer than one might
prefer, which can prevent expedited grace periods from completing in
a timely manner.  This in turn triggers a splat In nohz_full CPUs  One
could argue that long-looping code should be fixed, but on the other hand,
boot time is a bit special.

This commit therefore removes the splat.  Later commits will add the
splat back in, but in a way that removes false positives.
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_exp.h

@@ -508,7 +508,6 @@ static void synchronize_rcu_expedited_wait(void)
 				tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
 			}
 		}
-		WARN_ON_ONCE(1);
 	}
 
 	for (;;) {