vol: nuke() doesn't need to maintain next inode ptr

ni is unused on the true side of the if expression in nuke() - so
just don't bother populating it.

Caught by clang-analyzer

Change-Id: Idfd088f999f4c4bcdabc03b8b86636aa02541343
Reviewed-on: http://gerrit.openafs.org/9166
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>