DAFS: fssync online requires a partition name argument

DAFS: fssync online requires a partition name argument

fssync-debug online silently fails when run without a partition name.
Check for the required partition name on the server side and the client
side.  Report errors back to the client when the server side fails to
pre-attach the volume.

Change-Id: I454b202ac83893b1ee54d586a46d891773090a64
Reviewed-on: http://gerrit.openafs.org/5615
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>