Fix vcache/vnode mismatch in lookup for DARWIN

We have a vcache and need a vnode, so AFSTOV() is the right macro,
not VTOAFS().

It's kind of unfortunate that --enable-checking doesn't catch this.

Change-Id: I924649918dd0149b5fd7e4380b5f2e49d21fc8c4
Reviewed-on: http://gerrit.openafs.org/10702
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>