net: bridge: change unicast boolean to exact pkt_type

Remove the unicast flag and introduce an exact pkt_type. That would help us
for the upcoming per-port multicast flood flag and also slightly reduce the
tests in the input fast path.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>