NFSv4.1: new flag for state renewal check

Data servers not sharing a session with the mount MDS always have an empty
cl_superblocks list.
Replace the cl_superblocks empty list check to see if it is time to shut down
renewd with the NFS_CS_STOP_RENEW bit which is not set by such a data server.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>