SUNRPC: split client creation routine into setup and registration

This helper moves all "registration" code to the new rpc_client_register()
helper.
This helper will be used later in the series to synchronize against PipeFS
MOUNT/UMOUNT events.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>