vnet: add CURVNET_ASSERT_SET

Reviewed by:	kp
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D34312

sys/net/vnet.h

@@ -238,6 +238,10 @@ void vnet_log_recursion(struct vnet *, const char *, int);
 	curvnet = saved_vnet;
 #endif /* VNET_DEBUG */
 
+#define	CURVNET_ASSERT_SET()						\
+	VNET_ASSERT(curvnet != NULL, ("vnet is not set at %s:%d %s()",  \
+	    __FILE__, __LINE__, __func__))
+
 extern struct vnet *vnet0;
 #define	IS_DEFAULT_VNET(arg)	((arg) == vnet0)