nbd: flags is a u32 variable

The flags variable is used as u32 variable. This patch changes the type
to be u32.
Signed-off-by: NMarkus Pargmann <mpa@pengutronix.de>
Signed-off-by: NJens Axboe <axboe@fb.com>

drivers/block/nbd.c

@@ -41,7 +41,7 @@
 #include <linux/nbd.h>
 
 struct nbd_device {
-	int flags;
+	u32 flags;
 	struct socket * sock;	/* If == NULL, device is not ready, yet	*/
 	int magic;