vos: cross-device link error message

Print a better diagnostic message for cross-device link errors, which
happens when a clone volume is not in the same partition as the
parent read-write volume.

Change-Id: If58284a1dc53f8264fb17757f37c627fc2a378db
Reviewed-on: http://gerrit.openafs.org/10850
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>