net: linux/skbuff.h: combine SKB_EXTENSIONS + KCOV handling

The previous Kconfig patch led to some other build errors as
reported by the 0day bot and my own overnight build testing.

These are all in <linux/skbuff.h> when KCOV is enabled but
SKB_EXTENSIONS is not enabled, so fix those by combining those conditions
in the header file.

Fixes: 6370cc3 ("net: add kcov handle to skb extensions")
Fixes: 85ce50d ("net: kcov: don't select SKB_EXTENSIONS when there is no NET")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Aleksandr Nogikh <nogikh@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20201116212108.32465-1-rdunlap@infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -4608,7 +4608,7 @@ static inline void skb_reset_redirect(struct sk_buff *skb)
 #endif
 }
 
-#ifdef CONFIG_KCOV
+#if IS_ENABLED(CONFIG_KCOV) && IS_ENABLED(CONFIG_SKB_EXTENSIONS)
 static inline void skb_set_kcov_handle(struct sk_buff *skb,
 				       const u64 kcov_handle)
 {
@@ -4636,7 +4636,7 @@ static inline u64 skb_get_kcov_handle(struct sk_buff *skb)
 static inline void skb_set_kcov_handle(struct sk_buff *skb,
 				       const u64 kcov_handle) { }
 static inline u64 skb_get_kcov_handle(struct sk_buff *skb) { return 0; }
-#endif /* CONFIG_KCOV */
+#endif /* CONFIG_KCOV && CONFIG_SKB_EXTENSIONS */
 
 #endif	/* __KERNEL__ */
 #endif	/* _LINUX_SKBUFF_H */