Unix CM: Remove dead assignment in readdir

Unix CM: Remove dead assignment in readdir

len is zeroed at the start of the next block, so don't bother assigning
tlen to it if we're not going to do anything with it.

Caught by clang-analyzer

Change-Id: Ie1cd5a4d9dd96920018f054b6edfc18c569c9807
Reviewed-on: http://gerrit.openafs.org/9206
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>