netfilter: nf_tables: nft_exthdr: the presence return value should be little-endian

On big-endian machine, the returned register data when the exthdr is
present is not being compared correctly because little-endian is
assumed. The function nft_cmp_fast_mask(), called by nft_cmp_fast_eval()
and nft_cmp_fast_init(), calls cpu_to_le32().

The following dump also shows that little endian is assumed:

$ nft --debug=netlink add rule ip recordroute forward ip option rr exists counter
ip
  [ exthdr load ipv4 1b @ 7 + 0 present => reg 1 ]
  [ cmp eq reg 1 0x01000000 ]
  [ counter pkts 0 bytes 0 ]

Lastly, debug print in nft_cmp_fast_init() and nft_cmp_fast_eval() when
RR option exists in the packet shows that the comparison fails because
the assumption:

nft_cmp_fast_init:189 priv->sreg=4 desc.len=8 mask=0xff000000 data.data[0]=0x10003e0
nft_cmp_fast_eval:57 regs->data[priv->sreg=4]=0x1 mask=0xff000000 priv->data=0x1000000

v2: use nft_reg_store8() instead (Florian Westphal). Also to avoid the
    warnings reported by kernel test robot.

Fixes: dbb5281a ("netfilter: nf_tables: add support for matching IPv4 options")
Fixes: c078ca3b ("netfilter: nft_exthdr: Add support for existence check")
Signed-off-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_exthdr.c

@@ -44,7 +44,7 @@ static void nft_exthdr_ipv6_eval(const struct nft_expr *expr,
 
 	err = ipv6_find_hdr(pkt->skb, &offset, priv->type, NULL, NULL);
 	if (priv->flags & NFT_EXTHDR_F_PRESENT) {
-		*dest = (err >= 0);
+		nft_reg_store8(dest, err >= 0);
 		return;
 	} else if (err < 0) {
 		goto err;
@@ -141,7 +141,7 @@ static void nft_exthdr_ipv4_eval(const struct nft_expr *expr,
 
 	err = ipv4_find_option(nft_net(pkt), skb, &offset, priv->type);
 	if (priv->flags & NFT_EXTHDR_F_PRESENT) {
-		*dest = (err >= 0);
+		nft_reg_store8(dest, err >= 0);
 		return;
 	} else if (err < 0) {
 		goto err;