rcutorture: Suppress forward-progress complaints during early boot

Some larger systems can take in excess of 50 seconds to complete their
early boot initcalls prior to spawing init.  This does not in any way
help the forward-progress judgments of built-in rcutorture (when
rcutorture is built as a module, the insmod or modprobe command normally
cannot happen until some time after boot completes).  This commit
therefore suppresses such complaints until about the time that init
is spawned.

This also includes a fix to a stupid error located by kbuild test robot.

[ paulmck: Apply kbuild test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[ paulmck: Fix to nohz_full slow-expediting recovery logic, per bpetkov. ]
[ paulmck: Restrict splat to CONFIG_PREEMPT_RT=y kernels and simplify. ]
Tested-by: Borislav Petkov <bp@alien8.de>

include/linux/rcutree.h

@@ -54,6 +54,7 @@ void exit_rcu(void);
 void rcu_scheduler_starting(void);
 extern int rcu_scheduler_active __read_mostly;
 void rcu_end_inkernel_boot(void);
+bool rcu_inkernel_boot_has_ended(void);
 bool rcu_is_watching(void);
 #ifndef CONFIG_PREEMPTION
 void rcu_all_qs(void);