Linux: d_delete now takes a const argument

Linux: d_delete now takes a const argument

The d_delete dentry operation now takes a const argument.  Test for
this and define our function accordingly to avoid warnings.

Reviewed-on: http://gerrit.openafs.org/5335
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit def00ae8ba774f4d7290c29208484dda756dc0e0)

Change-Id: I128a721587f4963849652cb389472a550d81d267
Reviewed-on: http://gerrit.openafs.org/6097
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>