netfilter: nfnetlink_osf: rename nf_osf header file to nfnetlink_osf

The first client of the nf_osf.h userspace header is nft_osf, coming in
this batch, rename it to nfnetlink_osf.h as there are no userspace
clients for this yet, hence this looks consistent with other nfnetlink
subsystem.

Suggested-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>