netfilter: nf_tables: implement Passive OS fingerprint module in nft_osf

Add basic module functions into nft_osf.[ch] in order to implement OSF
module in nf_tables.

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>