SUNRPC: Never reuse the socket port after an xs_close()

If we call xs_close(), we're in one of two situations:
 - Autoclose, which means we don't expect to resend a request
 - bind+connect failed, which probably means the port is in use

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org