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)

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>
(cherry picked from commit 4b93c42513785d1094c5336b5c9cc4add1b89c5e)

Change-Id: I9849897ae69a426026f6d030ca4e50e8cd7066b2
Reviewed-on: http://gerrit.openafs.org/6762
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>