netfilter: xt_osf: fix xt_osf_remove_callback() return value

Return a negative error value.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>