nfsd: use nfs client rpc callback program

since commit ff7d9756
"nfsd: use static memory for callback program and stats"
do_probe_callback uses a static callback program
(NFS4_CALLBACK) rather than the one set in clp->cl_callback.cb_prog
as passed in by the client in setclientid (4.0)
or create_session (4.1).

This patches introduces rpc_create_args.prognumber that allows
overriding program->number when creating rpc_clnt.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>