NFS: Set NFS_CS_MIGRATION for NFSv4 mounts

NFS_CS_MIGRATION makes sense only for NFSv4 mounts.  Introduced by
commit 89652617 (NFS: Introduce "migration" mount option) Fri Sep 14
17:24:11 2012.

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