LU-18199 scripts: fix ksocklnd-config gateway selection logic

LU-18199 scripts: fix ksocklnd-config gateway selection logic

To avoid disruption of network connectivity during LNet start-up,
fix gateway selection logic in ksocklnd-config script to:
        1) select only pingable gateways
        2) prefer to select the gateway featured in the default route
           with the "dev" matching the interface being configured

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: I7fdfcf2d7e9acae18e9a2efb34a58c17d2887683
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56321
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>