netfilter: nft_meta: fix lack of validation of the input register

We have to validate that the input register is in the range of
allowed registers, otherwise we can take a incorrect register
value as input that may lead us to a crash.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_meta.c

@@ -239,6 +239,9 @@ static int nft_meta_init(const struct nft_ctx *ctx, const struct nft_expr *expr,
 		return err;
 
 	priv->sreg = ntohl(nla_get_be32(tb[NFTA_META_SREG]));
+	err = nft_validate_input_register(priv->sreg);
+	if (err < 0)
+		return err;
 
 	return 0;
 }