filer: adjust recommended mysql meta data type to LONGBLOB

fix https://github.com/chrislusf/seaweedfs/issues/966

weed/command/scaffold.go

@@ -75,7 +75,7 @@ dir = "."					# directory to store level db files
 #   dirhash     BIGINT         COMMENT 'first 64 bits of MD5 hash value of directory field',
 #   name        VARCHAR(1000)  COMMENT 'directory or file name',
 #   directory   TEXT           COMMENT 'full path to parent directory',
-#   meta        BLOB,
+#   meta        LONGBLOB,
 #   PRIMARY KEY (dirhash, name)
 # ) DEFAULT CHARSET=utf8;