netfilter: nft_set_pipapo: shrink data structures

The set uses a mix of 'int', 'unsigned int', and size_t.

The rule count limit is NFT_PIPAPO_RULE0_MAX, which cannot
exceed INT_MAX (a few helpers use 'int' as return type).

Add a compile-time assertion for this.

Replace size_t usage in structs with unsigned int or u8 where
the stored values are smaller.

Replace signed-int arguments for lengths with 'unsigned int'
where possible.

Last, remove lt_aligned member: its set but never read.

struct nft_pipapo_match 40 bytes -> 32 bytes
struct nft_pipapo_field 56 bytes -> 32 bytes

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>