f2fs: fix memory alignment to support 32bit

In 32bit system, 64-bits key breaks memory alignment.
This fixes the commit "f2fs: support 64-bits key in f2fs rb-tree node entry".
Reported-by: NNicolas Chauvet <kwizart@gmail.com>
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -619,7 +619,7 @@ struct rb_entry {
 			unsigned int len;	/* length of the entry */
 		};
 		unsigned long long key;		/* 64-bits key */
-	};
+	} __packed;
 };
 
 struct extent_info {