Expand GRN_API_ENTER/RETURN scope

grn_plugin_path() should be called in GRN_API_ENTER/RETURN scope
because grn_plugin_path() doesn't call GRN_API_ENTER/RETURN by itself.

Note that it is correct that grn_plugin_path() isn't called in
GRN_API_RETURN() argument. If it is called in GRN_API_RETURN()
argument, grn_plugin_path() is called in out of GRN_API_ENTER/RETURN
scope.