Calling vol:umount() would fail (#2558)

The volume returned by file.mount() could not be unmounted, because vol:umount() would fail with a cryptic error about the uncallable nature of the volume userdata object. This was due to the wrong metatable name being used for setting up the volume structure. The correct name, as registered elsewhere in file.c, is now used, and vol:umount() is callable.

app/modules/file.c

@@ -587,7 +587,7 @@ static int file_mount( lua_State *L )
 
   if (vol->vol = vfs_mount( ldrv, num )) {
     /* set its metatable */
-    luaL_getmetatable(L, "vfs.vol");
+    luaL_getmetatable(L, "file.vol");
     lua_setmetatable(L, -2);
     return 1;
   } else {