Do not corrupt volume linktable when special file already exists

Do not corrupt volume linktable when special file already exists

Upon volume creation, when a special file unexpectedly exists, e.g. due
to a failed cleanup, do not blindly assume that it is the link table.
Otherwise the latter's magic will get silently overwritten.

Change-Id: Iba05ae771e04272a3dab5534fcfc24cda5ee7df5
Reviewed-on: http://gerrit.openafs.org/1903
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>