netfilter: nf_tables: remove useless U8_MAX validation

After call nft_data_init, size is already validated and desc.len will
not exceed the sizeof(struct nft_data), i.e. 16 bytes. So it will never
exceed U8_MAX.

Furthermore, in nft_immediate_init, we forget to call nft_data_uninit
when desc.len exceeds U8_MAX, although this will not happen, but it's
a logical mistake.

Now remove these redundant validation introduced by commit 36b701fae12a
("netfilter: nf_tables: validate maximum value of u32 netlink attributes")

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>