netisr: fix compilation without VIMAGE

- Don't use bare vnet(4) definitions in the KASSERT, they aren't available
  on a kernel without VIMAGE.  Just through MPASS() here.  This is more of
  documenting assertion rather than an assertion that may actually fire on
  an unmodified kernel.
- V_netisr_enable is different to the rest of V_ prefixed globals.  On a
  kernel without VIMAGE it basically doesn't exist, instead of being
  present as a single instance.

Fixes:	a1be7978f1

sys/net/netisr.c

@@ -396,8 +396,7 @@ netisr_register(const struct netisr_handler *nhp)
 	 * Test that the requested registration is valid.
 	 */
 	CURVNET_ASSERT_SET();
-	KASSERT(IS_DEFAULT_VNET(curvnet), ("%s: curvnet %p is not vnet0 %p",
-	    __func__, curvnet, vnet0));
+	MPASS(IS_DEFAULT_VNET(curvnet));
 	KASSERT(nhp->nh_name != NULL,
 	    ("%s: nh_name NULL for %u", __func__, proto));
 	KASSERT(nhp->nh_handler != NULL,
@@ -457,8 +456,8 @@ netisr_register(const struct netisr_handler *nhp)
 		npwp->nw_qlimit = netisr_proto[proto].np_qlimit;
 	}
 
-	V_netisr_enable[proto] = 1;
 #ifdef VIMAGE
+	V_netisr_enable[proto] = 1;
 	VNET_LIST_RLOCK_NOSLEEP();
 	VNET_FOREACH(vnet_iter) {
 		if (vnet_iter == curvnet)