SUNRPC: Remove sock and inet fields from rpc_xprt

The "sock" and "inet" fields are socket-specific.  Move them to a private
data structure maintained entirely within net/sunrpc/xprtsock.c

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