LU-17455 scripts: add IPv6 support to ksocklnd-config

LU-17455 scripts: add IPv6 support to ksocklnd-config

Expand ksocklnd-config script to support IPv6.
For every interface listed as the argument, check if IPv6
address is configured and set up routing accordingly.
The change replicates existing behavior for IPv4:
   - if existing route is found for the interface,
     or skip_mr_routing is enabled, the script skips
     adding a new route and prints a warning
   - if default gateway is found on the same subnet,
     a source-based rule and route are added for the
     IP/interface using the gateway
   - if default gateway is not found, a source-based rule
     and a local route are added for the IP/interface

Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: I69e249f2858a201f1b108afa05cce9fdf4ee8c80
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54833
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>