vol: Allow VAllocVnode of specific vnodes

vol: Allow VAllocVnode of specific vnodes

Add parameters to VAllocVnode to allow the caller to specifiy the
vnode and unique numbers to use.  This will be used by the RW
replication code to keep vnode numbers in sync between the master
volume and the replicas.

Adapted from code by Vishal Powar and Derrick Brashear.

Change-Id: Ibaf79aad2b3e7a52802f5e01f7e4c7730c3f5090
Reviewed-on: http://gerrit.openafs.org/6672
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>