linux(4): Add net epoch assert to the linux_ifhwaddr, linux_ifflags

Now this functions are intended to use in the net epoch.

Reviewed by:		emaste, melifaro
Differential Revision:	https://reviews.freebsd.org/D38795

sys/compat/linux/linux.c

@@ -477,6 +477,8 @@ int
 linux_ifhwaddr(struct ifnet *ifp, struct l_sockaddr *lsa)
 {
 
+	NET_EPOCH_ASSERT();
+
 	if (IFP_IS_LOOP(ifp)) {
 		bzero(lsa, sizeof(*lsa));
 		lsa->sa_family = LINUX_ARPHRD_LOOPBACK;