LINUX: Check afs_lookup return code explicitly

Checking if the returned vcache is NULL or not is a bit of an indirect
way to check if an error occurred. Just check the return code itself,
to make sure we notice if any kind of error is reported.

Suggested by Chas Williams.

Reviewed-on: http://gerrit.openafs.org/11321
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 2edf5c0382385f898a017fd8e0e2429f8b2b3520)

Change-Id: I7e123ab1cf88570a6b18e438e01409ed7804e014
Reviewed-on: http://gerrit.openafs.org/11558
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>