libafs: fix vrequest leak in afs_lookup

Fix vrequest leak introduced in commit
9930567bcf9655d3f562b210b2dc4b4a99226691.

Thanks to Andrew Deason for finding this error.

Reviewed-on: http://gerrit.openafs.org/11337
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 51c97beb3f3bb68704d33e126561b8c2866ddab3)

Change-Id: If903c7863ac2ac6420a74e6bea1012c8cbf41913
Reviewed-on: http://gerrit.openafs.org/11364
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>