swsusp: Fix userland interface

Fix oops caused by 'cat /dev/snapshot', reported by Arkadiusz Miskiewicz,
and make it impossible to thaw tasks with the help of the swsusp userland
interface while there is a snapshot image ready to save.

Signed-off-by: Rafael J. Wysocki <[e-mail]>
Acked-by: Pavel Machek <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/power/user.c

···
 	ssize_t res;
 
 	data = filp->private_data;
+	if (!data->ready)
+		return -ENODATA;
 	res = snapshot_read_next(&data->handle, count);
 	if (res > 0) {
 		if (copy_to_user(buf, data_of(data->handle), res))
···
 		break;
 
 	case SNAPSHOT_UNFREEZE:
-		if (!data->frozen)
+		if (!data->frozen || data->ready)
 			break;
 		mutex_lock(&pm_mutex);
 		thaw_processes();