Linux 3.18: d_invalidate can no longer return an error

d_invalidate is now defined as void and does not have
a return value to check.

Reviewed-on: http://gerrit.openafs.org/11562
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit a42f01d5ebb13da575b3123800ee6990743155ab)

Change-Id: I8542404771c4a7962238efd9a53d7dfcf4011c96
Reviewed-on: http://gerrit.openafs.org/11569
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>