libafs: do not allow NULL creds for afs_CreateReq

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.

Change-Id: I05f694026ec72ab701160d9920e47c16cda46cd7
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>