netfilter: nf_osf: rename nf_osf.c to nfnetlink_osf.c

Rename nf_osf.c to nfnetlink_osf.c as we introduce nfnetlink_osf which is
the OSF infraestructure.

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