viced: Check vnode length on dir ops

viced: Check vnode length on dir ops

The commit aadf69eabb1962496fa93745ab560a5b48cacd61 added checks on
vnode length whenever we read or write from a vnode. Add the same
check on directory vnodes when we modify the directory (whenever
entries are added or deleted).

Reviewed-on: http://gerrit.openafs.org/4233
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 2578555d7e08131bf2fe4cdd0aa4b32567a76eb2)

Change-Id: I105f0dc7b480f7328205ae02929f6ca187aa962a
Reviewed-on: http://gerrit.openafs.org/5769
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>