prdb_check: check for continuation entries in owner chains

prdb_check: check for continuation entries in owner chains

Continuation entries may not be in owner chains. Fix the
comments in WalkOwnerChain (which were probably copied from
WalkNextChain) and add a check and error message for
continuation entries found on owner chains.

Change-Id: I8c49378478cf6a3d31317ff90a52fe1e74517dd3
Reviewed-on: http://gerrit.openafs.org/11751
Reviewed-by: Daria Phoebe Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>