header: fix broken headers for user space

__packed is only defined in kernel space, so we should use
__attribute__((packed)) for the code shared between kernel and user space.

Two __attribute() annotations are replaced with __attribute__() too.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/nbd.h b/include/linux/nbd.h
index bb58854..d146ca1 100644
--- a/include/linux/nbd.h
+++ b/include/linux/nbd.h
@@ -88,7 +88,7 @@
 	char handle[8];
 	__be64 from;
 	__be32 len;
-} __packed;
+} __attribute__((packed));
 
 /*
  * This is the reply packet that nbd-server sends back to the client after