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.

Change-Id: I61cc7304e9885ddaaebe96db3b12457cb6224420
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>