viced: Sanity check file link count during CoW

viced: Sanity check file link count during CoW

A few ihandle bugs in the past have caused the CopyOnWrite code to
open cached file handles for files which have been deleted. When we
CoW, both of the files we're dealing with had better actually be on
disk, so bail out and flag an error if either of them appear unlinked.

FIXES 131530

Change-Id: I478871bb1b9b43fc0161bb4a255dd7b77d2a28ae
Reviewed-on: http://gerrit.openafs.org/8839
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>