src: replace size_t by uint32_t in set/get interfaces

This patch breaks the ABI to shrink the size parameter from 8
to 4 bytes in x86_64. The maximum length of netlink attributes
is 2 bytes, so 4 bytes as attribute payload length should be
enough.

After this patch, size_t is only used in the nft_*_snprintf
interfaces.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>