Introduce new "guard" rule upon FDB entry creation.

It matches on src_mac, has valid bit unset, allow_pass_l2 set
and has a nop action.

Previously introduced "forward" rule matches on dst_mac, has valid
bit set, need_pass_l2 set and has a forward action.

With these rules, a packet will be offloaded only if FDB exists in both
directions (RX and TX).

Let's assume link partner sends a packet to VF1: src_mac = LP_MAC,
dst_mac = is VF1_MAC. Bridge adds FDB, two rules are created:
1. Guard rule matching on src_mac == LP_MAC
2. Forward rule matching on dst_mac == LP_MAC
Now VF1 responds with src_mac = VF1_MAC, dst_mac = LP_MAC. Before this
change, only one rule with dst_mac == LP_MAC would have existed, and the
packet would have been offloaded, meaning the bridge wouldn't add FDB in
the opposite direction. Now, the forward rule matches (dst_mac == LP_MAC),
but it has need_pass_l2 set an there is no guard rule with
src_mac == VF1_MAC, so the packet goes through slow-path and the bridge
adds FDB. Two rules are created:
1. Guard rule matching on src_mac == VF1_MAC
2. Forward rule matching on dst_mac == VF1_MAC
Further packets in both directions will be offloaded.

The same example is true in opposite direction (i.e. VF1 is the first to
send a packet out).
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Marcin Szycik <marcin.szycik@intel.com>
Signed-off-by: Wojciech Drewek <wojciech.drewek@intel.com>
Tested-by: Sujai Buvaneswaran <sujai.buvaneswaran@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>