vnet: make VNET_FOREACH() always be a loop

VNET_FOREACH() is a LIST_FOREACH if VIMAGE is set, but empty if it's
not. This means that users of the macro couldn't use 'continue' or
'break' as one would expect of a loop.

Change VNET_FOREACH() to be a loop in all cases (although one that is
fixed to one iteration if VIMAGE is not set).

Reviewed by:	karels, melifaro, glebius
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D35739

sys/net/vnet.h

@@ -414,7 +414,7 @@ do {									\
 #define	VNET_LIST_RUNLOCK()
 #define	VNET_LIST_RUNLOCK_NOSLEEP()
 #define	VNET_ITERATOR_DECL(arg)
-#define	VNET_FOREACH(arg)
+#define	VNET_FOREACH(arg)	for (int _vn = 0; _vn == 0; _vn++)
 
 #define	IS_DEFAULT_VNET(arg)	1
 #define	CRED_TO_VNET(cr)	NULL