ICMPv6: restore Route Information option decoding

In May 2001 commit ea3df10 implemented support for then-current I-D on
the Route Information ICMPv6 option using type 9. In June 2001 type 9
was assigned to a completely different ICMPv6 option (Source Address
List, RFC3122). In 2005 the I-D became RFC4191 with the same option
encoding and assigned option type 24.

Update a macro to fix decoding and add a test case for it (produced
with an unconfigured instance of a development build of OpenWrt).