Currently, there is no direct link from (struct symbol) to (struct menu).

It is still possible to access associated menus through the P_SYMBOL
property, because property::menu is the relevant menu entry, but it
results in complex code, as seen in get_symbol_str().

Use a linked list for simpler traversal of relevant menus.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>