DragonFlyBSD

As of now, kernel build without INVARIANTS fails - this fixes it:

diff --git a/sys/netinet/tcp_usrreq.c b/sys/netinet/tcp_usrreq.c
index 36d7ab2..c4c0973 100644
--- a/sys/netinet/tcp_usrreq.c
+++ b/sys/netinet/tcp_usrreq.c
@@ -732,7 +732,7 @@ tcp_usr_send(netmsg_t msg)
struct socket *so = msg->send.base.nm_so;
int flags = msg->send.nm_flags;
struct mbuf *m = msg->send.nm_m;
- struct mbuf *control = msg->send.nm_control;
+ struct mbuf *control __debugvar = msg->send.nm_control;
int error = 0;
struct inpcb *inp;
struct tcpcb *tp;