SUNRPC: Ensure the server closes sockets in a timely fashion

We want to ensure that connected sockets close down the connection when we
set XPT_CLOSE, so that we don't keep it hanging while cleaning up all the
stuff that is keeping a reference to the socket.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>