volser: Ignore duplicate file tags when restoring

volser: Ignore duplicate file tags when restoring

Currently, if the volserver encounters multiple file tags for a vnode
when restoring a volume dump, it will create extra vnodes which are
not recorded anywhere, and are leaked when the volume is removed.
Instead of doing that, ignore any duplicate file tags that are
encountered (which is what happens with other tags), and log a
warning.

Change-Id: I1ac76b0217096d013db355b899f1a53174d20eb5
Reviewed-on: http://gerrit.openafs.org/2531
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>