NFS: Allow walking nfs_client.cl_superblocks list outside client.c

We're about to move some fields from struct nfs_client to struct
nfs_server.  There is a many-to-one relationship between nfs_servers
and nfs_clients.  After these fields are moved to the nfs_server
struct, to visit all of the data in these fields that is owned by one
nfs_client, code will need to visit each nfs_server on the
cl_superblocks list for that nfs_client.

To serialize changes to the cl_superblocks list during these little
expeditions, protect the list with RCU.

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