linux-gbmc: Enable ipv6 multi-routing table

We don't really use it but it shares some code with kernel local route
generation. We are seeing weird issue without this flags:
https://bugzilla.kernel.org/show_bug.cgi?id=217639

which can break bmc network. With a separate local table ID we are not
seeing such issue.

Tested:

before:
>ip addr add 2002:a05:7303:f30b:fd01:: dev dummy0
>ip addr replace 2002:a05:7303:f30b:fd01:: dev dummy0
>ip -6 route show table local | grep 2002:a05:7303:f30b:fd01::

after:
>ip addr add 2002:a05:7303:f30b:fd01:: dev dummy0
>ip addr replace 2002:a05:7303:f30b:fd01:: dev dummy0
>ip -6 route show table local | grep 2002:a05:7303:f30b:fd01::
local 2002:a05:7303:f30b:fd01:: dev dummy0 proto kernel metric 0 pref medium

ssh worked and bmc networking is up after boot

https://fusion2.corp.google.com/ci/guitar/projects/UNSPECIFIED/activity/0d9a02bc-8fd9-41da-80b3-ad3792e9a0cf/overview

Google-Bug-Id: 333985313
Change-Id: I408d68f3a7d701903a4290a42b42f38fab2771be
Signed-off-by: Yuxiao Zhang <yuxiaozhang@google.com>
(cherry picked from commit 1ff52fdaf95bcaa2c0a1afed4b5c4a019b5f3cff)
diff --git a/recipes-kernel/linux/files/defconfig b/recipes-kernel/linux/files/defconfig
index 324e74f..ebf59d6 100644
--- a/recipes-kernel/linux/files/defconfig
+++ b/recipes-kernel/linux/files/defconfig
@@ -64,6 +64,7 @@
 CONFIG_IPV6_ROUTER_PREF=y
 CONFIG_IPV6_ROUTE_INFO=y
 CONFIG_IPV6_OPTIMISTIC_DAD=y
+CONFIG_IPV6_MULTIPLE_TABLES=y
 CONFIG_SYN_COOKIES=y
 CONFIG_PCPU_DEV_REFCNT=y
 CONFIG_ETHTOOL_NETLINK=y