ng_pipe: Remove node when all hooks are disconnected

This is the behavior described in the man page.

Signed-off-by: Martin Vahlensieck <git@academicsolutions.ch>

Discussed with:	glebius
Reviewed by:	markj
MFC after:	2 weeks
Pull Request:	https://github.com/freebsd/freebsd-src/pull/1181

sys/netgraph/ng_pipe.c

@@ -936,6 +936,7 @@ ngp_disconnect(hook_p hook)
 	struct hookinfo *const hinfo = NG_HOOK_PRIVATE(hook);
 	struct ngp_fifo *ngp_f;
 	struct ngp_hdr *ngp_h;
+	priv_p priv;
 
 	KASSERT(hinfo != NULL, ("%s: null info", __FUNCTION__));
 	hinfo->hook = NULL;
@@ -962,6 +963,12 @@ ngp_disconnect(hook_p hook)
 	if (hinfo->ber_p)
 		free(hinfo->ber_p, M_NG_PIPE);
 
+	/* Destroy the node if all hooks are disconnected */
+	priv = NG_NODE_PRIVATE(NG_HOOK_NODE(hook));
+
+	if (priv->upper.hook == NULL && priv->lower.hook == NULL)
+		ng_rmnode_self(NG_HOOK_NODE(hook));
+
 	return (0);
 }