rcu: Make SRCU mandatory

Kernels configured with CONFIG_PRINTK=n and CONFIG_SRCU=n get build
failures.  This causes trouble for deep embedded systems.  But given
that there are more than 25 instances of "select SRCU" in the kernel,
it is hard to believe that there are many kernels running in production
without SRCU.  This commit therefore makes SRCU mandatory.  The SRCU
Kconfig option remains for backwards compatibility, and will be removed
when it is no longer used.

[ paulmck: Update per kernel test robot feedback. ]

Reported-by: John Ogness <john.ogness@linutronix.de>
Reported-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: <linux-arch@vger.kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Reviewed-by: John Ogness <john.ogness@linutronix.de>

include/linux/srcu.h

@@ -47,11 +47,8 @@ int init_srcu_struct(struct srcu_struct *ssp);
 #include <linux/srcutiny.h>
 #elif defined(CONFIG_TREE_SRCU)
 #include <linux/srcutree.h>
-#elif defined(CONFIG_SRCU)
-#error "Unknown SRCU implementation specified to kernel configuration"
 #else
-/* Dummy definition for things like notifiers.  Actual use gets link error. */
-struct srcu_struct { };
+#error "Unknown SRCU implementation specified to kernel configuration"
 #endif
 
 void call_srcu(struct srcu_struct *ssp, struct rcu_head *head,
@@ -78,11 +75,7 @@ static inline void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
 }
 #endif /* CONFIG_NEED_SRCU_NMI_SAFE */
 
-#ifdef CONFIG_SRCU
 void srcu_init(void);
-#else /* #ifdef CONFIG_SRCU */
-static inline void srcu_init(void) { }
-#endif /* #else #ifdef CONFIG_SRCU */
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC