libafs: do not allow NULL creds for afs_CreateReq

Do not allow callers to pass a NULL cred to afs_CreateReq.  This
avoids setting the uid of zero in the vrequest when no cred is
passed.  Update callers to pass afs_osi_credp for an anonymous cred
when no cred is available.

Thanks to Andrew Deason for pointing out afs_osi_credp should be
used.

Reviewed-on: http://gerrit.openafs.org/11336
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: D Brashear <shadow@your-file-system.com>
(cherry picked from commit b7f1763652fb932ca5bd3f3351a48df11e58f2dc)

Change-Id: I0a7f17a05decdd0a9ef1ff72d7bf73153db95bed
Reviewed-on: http://gerrit.openafs.org/11365
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>