linux: Fix leaked dentry reference in the revalidate op

In one error case, we exit the function without release the
reference on the parent dentry.  This dangling reference can cause
an oops when the client is shut down.

Change-Id: I1632b581f6e7a557bb54a999dc1837c31b8381bf
Reviewed-on: http://gerrit.openafs.org/9802
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>