netfilter: nft_cmp: optimize comparison for 16-bytes

Allow up to 16-byte comparisons with a new cmp fast version. Use two
64-bit words and calculate the mask representing the bits to be
compared. Make sure the comparison is 64-bit aligned and avoid
out-of-bound memory access on registers.

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