netfilter: nf_tables: fix userdata length overflow

The NFT_USERDATA_MAXLEN is defined to 256, however we only have a u8
to store its size. Introduce a struct nft_userdata which contains a
length field and indicate its presence using a single bit in the rule.

The length field of struct nft_userdata is also a u8, however we don't
store zero sized data, so the actual length is udata->len + 1.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>