nfs41: Adjust max cache response size value

For the CREATE_SESSION attribute ca_maxresponsesize_cached, calculate
the value based on the rpc reply header size plus the maximum nfs compound
reply size.

Signed-off-by: Mike Sager <sager@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>