filter: Allow to create sk-unattached filters

Today, BPF filters are bind to sockets. Since BPF machine becomes handy
for other purposes, this patch allows to create unattached filter.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Samuel Pascua <pascua.samuel.14@gmail.com>