LINUX: eliminate unused variable warning

LINUX: eliminate unused variable warning

Commit c3bbf0b4444db88192eea4580ac9e9ca3de0d286 added routine
osi_TryEvictDentries and included new logic for D_INVALIDATE_IS_VOID.
Unfortunately, this new code path no longer uses dentry; it also should
have been made conditional at that time.

Wrap the declaration of dentry in #ifndef D_INVALIDATE_IS_VOID to
eliminate the unused variable warning.

Reviewed-on: https://gerrit.openafs.org/12505
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 19599b5ef5f7dff2741e13974692fe4a84721b59)

Change-Id: Ic15df733fcbccfaf9870ecd335bb2d549ab0d43d
Reviewed-on: https://gerrit.openafs.org/12513
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>