volser: log message for cross-device link errors

Add a log entry to the volume server to help diagnose those pesky
'Invalid cross-link device' errors returned by vos, which occur when
a clone volume is located in a different partition than the parent
read-write volume, or when a read-only volume is on the incorrect
partition on the server.

With this change, a new log entry is added when the volume server
fails to create a clone or a read-write volume because a volume with
the target volume id already exists on a different partition.  For a
clone volume, this would be a different partition than the
read-write volume. For a read-only volume, this would be a different
partition than indicated in the vldb.

Examples:

Volume foobar is on /vicepb, but foobar.backup is incorrectly on
partition /vicepa.

$ vos backup foobar
Failed to clone the volume 536870934
: Invalid cross-device link

VolserLog:
 VCreateVolume: volume 536870936 for parent 536870934 found on /vicepa; unable to create volume on /vicepb.
 1 Volser: Clone: Couldn't create new volume 536870936 for parent 536870934; clone aborted
 ...

The vldb indicates a read-only volume should be on /vicepa on a
remote site, but the actual volume is on /vicepb.

$ vos release xyzzy
Failed to create the ro volume: : Input/output error
The volume 536870921 could not be released to the following 1 sites:
	                             mantis /vicepa
VOLSER: release could not be completed
...

VolserLog on mantis:
 VCreateVolume: volume 536870922 for parent 536870921 found on /vicepb; unable to create volume on /vicepa.
 ...

Change-Id: Iaa471c46059d598a5095d59580e3b0b8ac6e1992
Reviewed-on: http://gerrit.openafs.org/10849
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>