bonding: 802.3ad replace MAC_ADDRESS_EQUAL with __agg_has_partner

Replace macro MAC_ADDRESS_EQUAL() for null_mac_addr checking with inline
function__agg_has_partner(). When MAC_ADDRESS_EQUAL() is verifiying
aggregator's partner mac addr with null_mac_addr, means that seeing if
aggregator has a valid partner or not. Using __agg_has_partner() makes it
more clear to understand.

In ad_port_selection_logic(), since aggregator->partner_system and
port->partner_oper.system has been compared first as a prerequisite, it is
safe to replace the upcoming MAC_ADDRESS_EQUAL() for null_mac_addr checking
with __agg_has_partner().

Delete null_mac_addr, which is not required anymore in bond_3ad.c, since
all references to it are gone.

Signed-off-by: Jones Syue <jonessyue@qnap.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Link: https://lore.kernel.org/r/SI2PR04MB5097BCA8FF2A2F03D9A5A3EEDC5A2@SI2PR04MB5097.apcprd04.prod.outlook.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>