printk: Tie printk_once / printk_deferred_once into .data.once for reset

In commit b1fca27 ("kernel debug: support resetting WARN*_ONCE")
we got the opportunity to reset state on the one shot messages,
without having to reboot.

However printk_once (printk_deferred_once) live in a different file
and didn't get the same kind of update/conversion, so they remain
unconditionally one shot, until the system is rebooted.

For example, we currently have:

  sched/rt.c: printk_deferred_once("sched: RT throttling activated\n");

..which could reasonably be tripped as someone is testing and tuning
a new system/workload and their task placements.  For consistency, and
to avoid reboots in the same vein as the original commit, we make these
two instances of _once the same as the WARN*_ONCE instances are.

Link: http://lkml.kernel.org/r/1555121491-31213-1-git-send-email-paul.gortmaker@windriver.com
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

Documentation/clearing-warn-once.txt

@@ -1,5 +1,5 @@
 
-WARN_ONCE / WARN_ON_ONCE only print a warning once.
+WARN_ONCE / WARN_ON_ONCE / printk_once only emit a message once.
 
 echo 1 > /sys/kernel/debug/clear_warn_once
 

include/linux/printk.h

@@ -347,7 +347,7 @@ extern int kptr_restrict;
 #ifdef CONFIG_PRINTK
 #define printk_once(fmt, ...)					\
 ({								\
-	static bool __print_once __read_mostly;			\
+	static bool __section(.data.once) __print_once;		\
 	bool __ret_print_once = !__print_once;			\
 								\
 	if (!__print_once) {					\
@@ -358,7 +358,7 @@ extern int kptr_restrict;
 })
 #define printk_deferred_once(fmt, ...)				\
 ({								\
-	static bool __print_once __read_mostly;			\
+	static bool __section(.data.once) __print_once;		\
 	bool __ret_print_once = !__print_once;			\
 								\
 	if (!__print_once) {					\