skbuff: replace a BUG_ON() with the new DEBUG_NET_WARN_ON_ONCE()

Very few drivers actually have Kconfig knobs for adding
-DDEBUG. 8 according to a quick grep, while there are
93 users of skb_checksum_none_assert(). Switch to the
new DEBUG_NET_WARN_ON_ONCE() to catch bad skbs.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220511172305.1382810-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -5048,9 +5048,7 @@ static inline void skb_forward_csum(struct sk_buff *skb)
  */
 static inline void skb_checksum_none_assert(const struct sk_buff *skb)
 {
-#ifdef DEBUG
-	BUG_ON(skb->ip_summed != CHECKSUM_NONE);
-#endif
+	DEBUG_NET_WARN_ON_ONCE(skb->ip_summed != CHECKSUM_NONE);
 }
 
 bool skb_partial_csum_set(struct sk_buff *skb, u16 start, u16 off);