SUNRPC: move saved socket callback functions to a private data structure

Move the three fields for saving socket callback functions out of the
rpc_xprt structure and into a private data structure maintained in
net/sunrpc/xprtsock.c.

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