rpc: share one xps between all backchannels

The spec allows backchannels for multiple clients to share the same tcp
connection.  When that happens, we need to use the same xprt for all of
them.  Similarly, we need the same xps.

This fixes list corruption introduced by the multipath code.

Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Acked-by: Trond Myklebust <trondmy@primarydata.com>

net/sunrpc/svc_xprt.c

@@ -136,6 +136,8 @@ static void svc_xprt_free(struct kref *kref)
 	/* See comment on corresponding get in xs_setup_bc_tcp(): */
 	if (xprt->xpt_bc_xprt)
 		xprt_put(xprt->xpt_bc_xprt);
+	if (xprt->xpt_bc_xps)
+		xprt_switch_put(xprt->xpt_bc_xps);
 	xprt->xpt_ops->xpo_free(xprt);
 	module_put(owner);
 }