netfilter: nf_tables: add optimized data comparison for small values

Add an optimized version of nft_data_cmp() that only handles values of to
4 bytes length.

This patch includes original Patrick McHardy's patch entitled (nf_tables:
inline nft_cmp_fast_eval() into main evaluation loop).

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