The routing table (RT) has been requiring gateway addresses not
to be included in the prefix of gateway and grantor RT entries.
This commit removes this requirement for gateway entries but
maintains it for grantor entries.

The motivation is to support default routes.
Without this commit, default routes are denied with log entries
similar to the ones below:

DYC/9: check_gateway_prefix(0.0.0.0/0): gateway address is in prefix, so gateway is not a neighbor
DYC/9: check_gateway_prefix(::/0): gateway address is in prefix, so gateway is not a neighbor

When the default IPv6 route is present, lookup_fe_from_lpm()
forwards multicast and link-local IPv6 packets according to
the default route instead of passing these packets to
other functional blocks.  This behavior breaks
the ND implementation in the LLS block.

This commit filters out multicast and link-local IPv6 packets
in extract_packet_info(), so other functional blocks always get
these packets.