LU-17331 tests: fix conf-sanity/30b fake NID generation

LU-17331 tests: fix conf-sanity/30b fake NID generation

It appears that "NID + 20" used by conf-sanity test_30b would
occasionally match the client NID and cause the test to fail.

Ensure the "fake" NID generated for OSS failover is not already
used by the cluster, and it also handles IPv6 NIDs.

Update code style in this subtest to match current standards.

Test-Parameters: trivial testlist=conf-sanity env=ONLY=30
Test-Parameters: trivial testlist=conf-sanity env=ONLY=30
Test-Parameters: trivial testlist=conf-sanity env=ONLY=30
Test-Parameters: trivial testlist=conf-sanity env=ONLY=30
Test-Parameters: trivial testlist=conf-sanity env=ONLY=30
Test-Parameters: trivial testlist=conf-sanity env=ONLY=30
Fixes: b91d5d4263 ("b=15253 fix conf-sanity 30b for non-tcp networks")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I119c8123193d2d0947ebbdbac5e5c7fb50d34ca5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55890
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>