SUNRPC: Move rpc_xprt socket connect fields into private data structure

Move the socket-specific connection management fields out of the generic
rpc_xprt structure 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>