nfs41: Backchannel: New find_client_with_session()

Finds the 'struct nfs_client' that matches the server's address, major
version number, and session ID.

Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>