NFS: Rename nfs41_call_sync_data as a common data structure

Clean up: rename nfs41_call_sync_data for use as a data structure
common to all NFSv4 minor versions.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>