netfilter: nf_tables: check if payload length is a power of 2

Add a check if payload's length is a power of 2 when selecting ops.
The fast ops were meant for well aligned loads, also this fixes a
small bug when using a length of 3 with some offsets which causes
only 1 byte to be loaded because the fast ops are chosen.
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_payload.c

@@ -135,7 +135,8 @@ nft_payload_select_ops(const struct nft_ctx *ctx,
 	if (len == 0 || len > FIELD_SIZEOF(struct nft_data, data))
 		return ERR_PTR(-EINVAL);
 
-	if (len <= 4 && IS_ALIGNED(offset, len) && base != NFT_PAYLOAD_LL_HEADER)
+	if (len <= 4 && is_power_of_2(len) && IS_ALIGNED(offset, len) &&
+	    base != NFT_PAYLOAD_LL_HEADER)
 		return &nft_payload_fast_ops;
 	else
 		return &nft_payload_ops;