net: add CONFIG_DEBUG_NET

This config option enables network debugging checks.

This patch adds DEBUG_NET_WARN_ON_ONCE(cond)
Note that this is not a replacement for WARN_ON_ONCE(cond)
as (cond) is not evaluated if CONFIG_DEBUG_NET is not set.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_debug.h

@@ -148,4 +148,10 @@ do {								\
 #endif
 
 
+#if defined(CONFIG_DEBUG_NET)
+#define DEBUG_NET_WARN_ON_ONCE(cond) (void)WARN_ON_ONCE(cond)
+#else
+#define DEBUG_NET_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
+#endif
+
 #endif	/* _LINUX_NET_DEBUG_H */

net/Kconfig.debug

@@ -17,3 +17,10 @@ config NET_NS_REFCNT_TRACKER
 	help
 	  Enable debugging feature to track netns references.
 	  This adds memory and cpu costs.
+
+config DEBUG_NET
+	bool "Add generic networking debug"
+	depends on DEBUG_KERNEL
+	help
+	  Enable extra sanity checks in networking.
+	  This is mostly used by fuzzers, but is safe to select.