dell_rbu: use scnprintf() instead of less secure sprintf()

Reading 0 bytes from /sys/devices/platform/dell_rbu/image_type or
/sys/devices/platform/dell_rbu/packet_size by an ordinary user causes an
oops.
Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/firmware/dell_rbu.c

@@ -576,7 +576,7 @@ static ssize_t read_rbu_image_type(struct kobject *kobj,
 {
 	int size = 0;
 	if (!pos)
-		size = sprintf(buffer, "%s\n", image_type);
+		size = scnprintf(buffer, count, "%s\n", image_type);
 	return size;
 }
 
@@ -648,7 +648,7 @@ static ssize_t read_rbu_packet_size(struct kobject *kobj,
 	int size = 0;
 	if (!pos) {
 		spin_lock(&rbu_data.lock);
-		size = sprintf(buffer, "%lu\n", rbu_data.packetsize);
+		size = scnprintf(buffer, count, "%lu\n", rbu_data.packetsize);
 		spin_unlock(&rbu_data.lock);
 	}
 	return size;