vol: allow clones of readonly volumes

vol: allow clones of readonly volumes

allow writing of data where it's not user data we're changing
(e.g. allow a vnode to be marked cloned in the vnode index)

Change-Id: If3338ab0474ddbfe895b705217d61c054c4cb696
Reviewed-on: http://gerrit.openafs.org/6251
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>