netfilter: bridge: make NF_TABLES_BRIDGE tristate

The new nft_meta_bridge code fails to link as built-in when NF_TABLES
is a loadable module.

net/bridge/netfilter/nft_meta_bridge.o: In function `nft_meta_bridge_get_eval':
nft_meta_bridge.c:(.text+0x1e8): undefined reference to `nft_meta_get_eval'
net/bridge/netfilter/nft_meta_bridge.o: In function `nft_meta_bridge_get_init':
nft_meta_bridge.c:(.text+0x468): undefined reference to `nft_meta_get_init'
nft_meta_bridge.c:(.text+0x49c): undefined reference to `nft_parse_register'
nft_meta_bridge.c:(.text+0x4cc): undefined reference to `nft_validate_register_store'
net/bridge/netfilter/nft_meta_bridge.o: In function `nft_meta_bridge_module_exit':
nft_meta_bridge.c:(.exit.text+0x14): undefined reference to `nft_unregister_expr'
net/bridge/netfilter/nft_meta_bridge.o: In function `nft_meta_bridge_module_init':
nft_meta_bridge.c:(.init.text+0x14): undefined reference to `nft_register_expr'
net/bridge/netfilter/nft_meta_bridge.o:(.rodata+0x60): undefined reference to `nft_meta_get_dump'
net/bridge/netfilter/nft_meta_bridge.o:(.rodata+0x88): undefined reference to `nft_meta_set_eval'

This can happen because the NF_TABLES_BRIDGE dependency itself is just a
'bool'. Make the symbol a 'tristate' instead so Kconfig can propagate the
dependencies correctly.

Fixes: 30e103fe24de ("netfilter: nft_meta: move bridge meta keys into nft_meta_bridge")
Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/bridge/netfilter/Kconfig

···
 menuconfig NF_TABLES_BRIDGE
 	depends on BRIDGE && NETFILTER && NF_TABLES
 	select NETFILTER_FAMILY_BRIDGE
-	bool "Ethernet Bridge nf_tables support"
+	tristate "Ethernet Bridge nf_tables support"
 
 if NF_TABLES_BRIDGE
 

net/netfilter/nft_chain_filter.c

···
 static inline void nft_chain_filter_inet_fini(void) {}
 #endif /* CONFIG_NF_TABLES_IPV6 */
 
-#ifdef CONFIG_NF_TABLES_BRIDGE
+#if IS_ENABLED(CONFIG_NF_TABLES_BRIDGE)
 static unsigned int
 nft_do_chain_bridge(void *priv,
 		    struct sk_buff *skb,

net/netfilter/nft_meta.c

···
 	if (tb[NFTA_META_DREG] && tb[NFTA_META_SREG])
 		return ERR_PTR(-EINVAL);
 
-#if defined(CONFIG_NF_TABLES_BRIDGE) && IS_MODULE(CONFIG_NFT_BRIDGE_META)
+#if IS_ENABLED(CONFIG_NF_TABLES_BRIDGE) && IS_MODULE(CONFIG_NFT_BRIDGE_META)
 	if (ctx->family == NFPROTO_BRIDGE)
 		return ERR_PTR(-EAGAIN);
 #endif