l2_packet: Improve bridge workaround RX processing

l2_packet: Improve bridge workaround RX processing

It was possible for the packet socket on the bridge interface to receive
own transmitted frames between the bridge and non-bridge sockets
receiving the same incoming frame from a foreign host. This resulted in
the hash checksum validation step failing to notice a duplicate RX due
to the own frame updating the store hash value.

The own frame did get dropping in RX EAPOL processing, but that was too
late to address the issue with duplicate RX. Fix this by dropping own
frames already in l2_packet layer before checking and updating the last
RX hash value.

Signed-off-by: Jouni Malinen <j@w1.fi>