lib/vsprintf: declare no_hash_pointers in sprintf.h

Sparse is not happy to see non-static variable without declaration:
lib/vsprintf.c:61:6: warning: symbol 'no_hash_pointers' was not declared. 
Should it be static?

Declare respective variable in the sprintf.h.  With this, add a comment to
discourage its use if no real need.

Link: https://lkml.kernel.org/r/20230814163344.17429-3-andriy.shevchenko@linux.intel.comSigned-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Marco Elver <elver@google.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/sprintf.h

@@ -20,6 +20,8 @@ __printf(2, 0) const char *kvasprintf_const(gfp_t gfp, const char *fmt, va_list
 __scanf(2, 3) int sscanf(const char *, const char *, ...);
 __scanf(2, 0) int vsscanf(const char *, const char *, va_list);
 
+/* These are for specific cases, do not use without real need */
+extern bool no_hash_pointers;
 int no_hash_pointers_enable(char *str);
 
 #endif	/* _LINUX_KERNEL_SPRINTF_H */

lib/test_printf.c

@@ -42,8 +42,6 @@ KSTM_MODULE_GLOBALS();
 static char *test_buffer __initdata;
 static char *alloced_buffer __initdata;
 
-extern bool no_hash_pointers;
-
 static int __printf(4, 0) __init
 do_test(int bufsize, const char *expect, int elen,
 	const char *fmt, va_list ap)

mm/kfence/report.c

@@ -13,6 +13,7 @@
 #include <linux/printk.h>
 #include <linux/sched/debug.h>
 #include <linux/seq_file.h>
+#include <linux/sprintf.h>
 #include <linux/stacktrace.h>
 #include <linux/string.h>
 #include <trace/events/error_report.h>
@@ -26,8 +27,6 @@
 #define ARCH_FUNC_PREFIX ""
 #endif
 
-extern bool no_hash_pointers;
-
 /* Helper function to either print to a seq_file or to console. */
 __printf(2, 3)
 static void seq_con_printf(struct seq_file *seq, const char *fmt, ...)