LINUX: don't cache negative entries for dynroot

LINUX: don't cache negative entries for dynroot

The dynroot volume lacks any callbacks that would invalidate the directory
or change the data version.  Further, the data version for the dynroot
is only updated for when a new cell is found or added (a positive lookup).

Change-Id: If0b022933de7335d3d94aafc77c50b85b99f4116
Reviewed-on: http://gerrit.openafs.org/12140
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>