printk: Use scnprintf() to print the message about the dropped messages on a console

Use scnprintf() for printing the message about dropped messages on
a console. It returns the really written length of the message.
It prevents potential buffer overflow when the returned length is
later used to copy the buffer content.

Note that the previous code was safe because the scratch buffer was
big enough and the message always fit in. But scnprintf() makes
it more safe, definitely.
Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1530570 ("Memory - corruptions")
Fixes: c4fcc617 ("printk: introduce console_prepend_dropped() for dropped messages")
Link: https://lore.kernel.org/r/202301131544.D9E804CCD@keescookReviewed-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230117161031.15499-1-pmladek@suse.com

kernel/printk/printk.c

@@ -2716,7 +2716,7 @@ static void console_prepend_dropped(struct printk_message *pmsg, unsigned long d
 	char *outbuf = &pbufs->outbuf[0];
 	size_t len;
 
-	len = snprintf(scratchbuf, scratchbuf_sz,
+	len = scnprintf(scratchbuf, scratchbuf_sz,
 		       "** %lu printk messages dropped **\n", dropped);
 
 	/*