[NETFILTER]: Consolidate nf_sockopt and compat_nf_sockopt

Both lookup the nf_sockopt_ops object to call the get/set callbacks
from, but they perform it in a completely similar way.

Introduce the helper for finding the ops.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>