nbd: make device_attribute const

Make this const as is is only passed as an argument to the
function device_create_file and device_remove_file and the corresponding
arguments are of type const.
Done using Coccinelle

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -165,7 +165,7 @@ static ssize_t pid_show(struct device *dev,
 	return sprintf(buf, "%d\n", task_pid_nr(nbd->task_recv));
 }
 
-static struct device_attribute pid_attr = {
+static const struct device_attribute pid_attr = {
 	.attr = { .name = "pid", .mode = S_IRUGO},
 	.show = pid_show,
 };