netfilter: arptables: allow xtables-nft only builds

Allows to build kernel that supports the arptables mangle target
via nftables' compat infra but without the arptables get/setsockopt
interface or the old arptables filter interpreter.

IOW, setting IP_NF_ARPFILTER=n will break arptables-legacy, but
arptables-nft will continue to work as long as nftables compat
support is enabled.
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Phil Sutter <phil@nwl.cc>

net/ipv4/netfilter/Kconfig

@@ -309,36 +309,34 @@ endif # IP_NF_IPTABLES
 
 # ARP tables
 config IP_NF_ARPTABLES
-	tristate "ARP tables support"
-	select NETFILTER_XTABLES
-	select NETFILTER_FAMILY_ARP
-	depends on NETFILTER_ADVANCED
-	help
-	  arptables is a general, extensible packet identification framework.
-	  The ARP packet filtering and mangling (manipulation)subsystems
-	  use this: say Y or M here if you want to use either of those.
-
-	  To compile it as a module, choose M here.  If unsure, say N.
+	tristate
 
-if IP_NF_ARPTABLES
+config NFT_COMPAT_ARP
+	tristate
+	depends on NF_TABLES_ARP && NFT_COMPAT
+	default m if NFT_COMPAT=m
+	default y if NFT_COMPAT=y
 
 config IP_NF_ARPFILTER
-	tristate "ARP packet filtering"
+	tristate "arptables-legacy packet filtering support"
+	select IP_NF_ARPTABLES
 	help
 	  ARP packet filtering defines a table `filter', which has a series of
 	  rules for simple ARP packet filtering at local input and
-	  local output.  On a bridge, you can also specify filtering rules
-	  for forwarded ARP packets. See the man page for arptables(8).
+	  local output.  This is only needed for arptables-legacy(8).
+	  Neither arptables-nft nor nftables need this to work.
 
 	  To compile it as a module, choose M here.  If unsure, say N.
 
 config IP_NF_ARP_MANGLE
 	tristate "ARP payload mangling"
+	depends on IP_NF_ARPTABLES || NFT_COMPAT_ARP
 	help
 	  Allows altering the ARP packet payload: source and destination
 	  hardware and network addresses.
 
-endif # IP_NF_ARPTABLES
+	  This option is needed by both arptables-legacy and arptables-nft.
+	  It is not used by nftables.
 
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