Assert that ifnet_detach_sxlock is held where needed.

Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
Reviewed by:	kp
Differential Revision:	https://reviews.freebsd.org/D41770

sys/net/if.c

@@ -1128,6 +1128,8 @@ if_detach_internal(struct ifnet *ifp, bool vmove)
 	shutdown = VNET_IS_SHUTTING_DOWN(ifp->if_vnet);
 #endif
 
+	sx_assert(&ifnet_detach_sxlock, SX_XLOCKED);
+
 	/*
 	 * At this point we know the interface still was on the ifnet list
 	 * and we removed it so we are in a stable state.