netfilter: nf_tables: add NFT_REG32_NUM

Add a definition including the maximum number of 32-bits registers that
are used a scratchpad memory area to store data.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -105,6 +105,8 @@ struct nft_data {
 	};
 } __attribute__((aligned(__alignof__(u64))));
 
+#define NFT_REG32_NUM		20
+
 /**
  *	struct nft_regs - nf_tables register set
  *
@@ -115,7 +117,7 @@ struct nft_data {
  */
 struct nft_regs {
 	union {
-		u32			data[20];
+		u32			data[NFT_REG32_NUM];
 		struct nft_verdict	verdict;
 	};
 };