[NET]: Add a dev_ioctl() fallback to sock_ioctl()

Currently all network protocols need to call dev_ioctl as the default
fallback in their ioctl implementations.  This patch adds a fallback
to dev_ioctl to sock_ioctl if the protocol returned -ENOIOCTLCMD.
This way all the procotol ioctl handlers can be simplified and we don't
need to export dev_ioctl.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -1859,7 +1859,7 @@ static int unix_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
 		}
 
 		default:
-			err = dev_ioctl(cmd, (void __user *)arg);
+			err = -ENOIOCTLCMD;
 			break;
 	}
 	return err;