ndp: correctness fix for BPF filter

ndp: correctness fix for BPF filter

Note that AF_PACKET sockets start receiving packets as soon as they are
created.  Thus, a packet can arrive between the creation of the socket
and the time the real filter is installed. Fix this using the same
technique as used in libpcap, i.e. by installing a drop-all filter,
removing any packets from the socket, then installing the real filter
later (an atomic operation which replaces the drop filter).

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/319
Signed-off-by: Álvaro Fernández Rojas <[email protected]>