LU-18199 scripts: fix to always add local route

LU-18199 scripts: fix to always add local route

To avoid disruption of network connectivity during LNet start-up,
fix gateway selection logic in ksocklnd-config script so that
the local route is added along with the default route to the
custom routing table created by this script.

Another change in this patch makes sure that link-local IPv6
addresses are ignored, because they are ignored by LNet anyway

Fixes: 7f60b2b55 ("LU-17006 lnet: set up routes for going across subnets")
Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: I341a40268298709c772bad84ddf4a4fae645b48f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56819
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>