attr: Fix memory leak for list and get operation

attr_get and attr_list were allocation GString but they were calling
g_string_free with second argument set to FALSE, what means GLib
was returning character data to caller (see g_string_free
for complete documentation) instead of freeing them. This return
value was ignored by attr_* functions so memory leaked.

Patch to fix this problem is simple - just set second argument of
g_string_free to TRUE so GLib takes care of freeing all data.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>