net: bridge: enfore alignment for ethernet address

The eth_addr member is passed to ether_addr functions that require
2-byte alignment, therefore the member must be properly aligned
to avoid unaligned accesses.

The problem is in place since the initial merge of multicast to unicast:
commit 6db6f0ea bridge: multicast to unicast

Fixes: 6db6f0ea ("bridge: multicast to unicast")
Cc: Roopa Prabhu <roopa@cumulusnetworks.com>
Cc: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Thomas Martitz <t.martitz@avm.de>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>