serial8250: sanity check nr_uarts on all paths.

I had 8250.nr_uarts=16 in the boot line of a test kernel and I had a weird
mysterious crash in sysfs.  After taking an in-depth look I realized that
CONFIG_SERIAL_8250_NR_UARTS was set to 4 and I was walking off the end of
the serial8250_ports array.

Ouch!!!

Don't let this happen to someone else.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Alan Cox <alan@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/8250.c

@@ -2623,6 +2623,9 @@ static struct console serial8250_console = {
 
 static int __init serial8250_console_init(void)
 {
+	if (nr_uarts > UART_NR)
+		nr_uarts = UART_NR;
+
 	serial8250_isa_init_ports();
 	register_console(&serial8250_console);
 	return 0;