printk: fix integer overflow in setup_log_buf()

The way we calculate logbuf free space percentage overflows signed
integer:

	int free;

	free = __LOG_BUF_LEN - log_next_idx;
	pr_info("early log buf free: %u(%u%%)\n",
		free, (free * 100) / __LOG_BUF_LEN);

We support LOG_BUF_LEN of up to 1<<25 bytes. Since setup_log_buf() is
called during early init, logbuf is mostly empty, so

	__LOG_BUF_LEN - log_next_idx

is close to 1<<25. Thus when we multiply it by 100, we overflow signed
integer value range: 100 is 2^6 + 2^5 + 2^2.

Example, booting with LOG_BUF_LEN 1<<25 and log_buf_len=2G
boot param:

[    0.075317] log_buf_len: -2147483648 bytes
[    0.075319] early log buf free: 33549896(-28%)

Make "free" unsigned integer and use appropriate printk() specifier.

Link: http://lkml.kernel.org/r/[email protected]
To: Steven Rostedt <[email protected]>
Cc: [email protected]
Cc: Sergey Senozhatsky <[email protected]>
Signed-off-by: Sergey Senozhatsky <[email protected]>
Signed-off-by: Petr Mladek <[email protected]>

kernel/printk/printk.c

@@ -1098,7 +1098,7 @@ void __init setup_log_buf(int early)
 {
 	unsigned long flags;
 	char *new_log_buf;
-	int free;
+	unsigned int free;
 
 	if (log_buf != __log_buf)
 		return;