f2fs: add symbol to avoid any confusion with tools

This patch adds MAX_VOLUME_NAME to sync with f2fs-tools.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

include/linux/f2fs_fs.h

@@ -51,6 +51,7 @@
 #define MAX_ACTIVE_DATA_LOGS	8
 
 #define VERSION_LEN	256
+#define MAX_VOLUME_NAME		512
 
 /*
  * For superblock
@@ -84,7 +85,7 @@ struct f2fs_super_block {
 	__le32 node_ino;		/* node inode number */
 	__le32 meta_ino;		/* meta inode number */
 	__u8 uuid[16];			/* 128-bit uuid for volume */
-	__le16 volume_name[512];	/* volume name */
+	__le16 volume_name[MAX_VOLUME_NAME];	/* volume name */
 	__le32 extension_count;		/* # of extensions below */
 	__u8 extension_list[F2FS_MAX_EXTENSION][8];	/* extension array */
 	__le32 cp_payload;