printk: Convert a use of sprintf to snprintf in console_unlock

When CONFIG_PRINTK is disabled (e.g. when building allnoconfig), clang
warns:

../kernel/printk/printk.c:2416:10: warning: 'sprintf' will always
overflow; destination buffer has size 0, but format string expands to at
least 33 [-Wfortify-source]
			len = sprintf(text,
			      ^
1 warning generated.

It is not wrong; text has a zero size when CONFIG_PRINTK is disabled
because LOG_LINE_MAX and PREFIX_MAX are both zero. Change to snprintf so
that this case is explicitly handled without any risk of overflow.

Link: ClangBuiltLinux#846
Link: llvm/llvm-project@6d485ff
Link: http://lkml.kernel.org/r/[email protected]
Cc: Steven Rostedt <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Nathan Chancellor <[email protected]>
Reviewed-by: Sergey Senozhatsky <[email protected]>
Signed-off-by: Petr Mladek <[email protected]>

Author: nathanchance

kernel/printk/printk.c

@@ -2413,9 +2413,9 @@ void console_unlock(void)
 		printk_safe_enter_irqsave(flags);
 		raw_spin_lock(&logbuf_lock);
 		if (console_seq < log_first_seq) {
-			len = sprintf(text,
-				      "** %llu printk messages dropped **\n",
-				      log_first_seq - console_seq);
+			len = snprintf(text, sizeof(text),
+				       "** %llu printk messages dropped **\n",
+				       log_first_seq - console_seq);
 
 			/* messages are gone, move to first one */
 			console_seq = log_first_seq;