s390/qeth: Split memcpy() of struct qeth_ipacmd_addr_change flexible array

To work around a misbehavior of the compiler's ability to see into
composite flexible array structs (as detailed in the coming memcpy()
hardening series[1]), split the memcpy() of the header and the payload
so no false positive run-time overflow warning will be generated.

[1] https://lore.kernel.org/linux-hardening/20220901065914.1417829-2-keescook@chromium.org/

Cc: Wenjia Zhang <wenjia@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://lore.kernel.org/r/20220927003953.1942442-1-keescook@chromium.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/s390/net/qeth_l2_main.c

@@ -1530,8 +1530,8 @@ static void qeth_addr_change_event(struct qeth_card *card,
 	else
 		INIT_DELAYED_WORK(&data->dwork, qeth_l2_dev2br_worker);
 	data->card = card;
-	memcpy(&data->ac_event, hostevs,
-			sizeof(struct qeth_ipacmd_addr_change) + extrasize);
+	data->ac_event = *hostevs;
+	memcpy(data->ac_event.entry, hostevs->entry, extrasize);
 	queue_delayed_work(card->event_wq, &data->dwork, 0);
 }