LU-10391 socklnd: support IPv6 in ksocknal_ip2index()

LU-10391 socklnd: support IPv6 in ksocknal_ip2index()

ksocknal_ip2index() can now find the interface index for an IPv6
address.

Test-Parameters: trivial testlist=sanity-lnet
Test-Parameters: serverversion=2.12 serverdistro=el7.9 testlist=runtests
Test-Parameters: clientversion=2.12 testlist=runtests
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Idd6bee5c9db417b05f8208ab5ab309f4c8404d54
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48570
Reviewed-by: jsimmons <jsimmons@infradead.org>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>