netfilter: nft_osf: usage from output path is not valid

The nft_osf extension, like xt_osf, is not supported from the output
path.

Fixes: b96af92d ("netfilter: nf_tables: implement Passive OS fingerprint module in nft_osf")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>