net: asix: fix fortify warning

When compiling with gcc version 14.0.0 20231129 (experimental) and
CONFIG_FORTIFY_SOURCE=y, I've noticed the following warning:

...
In function 'fortify_memcpy_chk',
    inlined from 'ax88796c_tx_fixup' at drivers/net/ethernet/asix/ax88796c_main.c:287:2:
./include/linux/fortify-string.h:588:25: warning: call to '__read_overflow2_field'
declared with attribute warning: detected read beyond size of field (2nd parameter);
maybe use struct_group()? [-Wattribute-warning]
  588 |                         __read_overflow2_field(q_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...

This call to 'memcpy()' is interpreted as an attempt to copy TX_OVERHEAD
(which is 8) bytes from 4-byte 'sop' field of 'struct tx_pkt_info' and
thus overread warning is issued. Since we actually want to copy both
'sop' and 'seg' fields at once, use the convenient 'struct_group()' here.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Łukasz Stelmach <l.stelmach@samsung.com>
Link: https://lore.kernel.org/r/20231211090535.9730-1-dmantipov@yandex.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/asix/ax88796c_main.c

@@ -284,7 +284,7 @@ ax88796c_tx_fixup(struct net_device *ndev, struct sk_buff_head *q)
 	ax88796c_proc_tx_hdr(&info, skb->ip_summed);
 
 	/* SOP and SEG header */
-	memcpy(skb_push(skb, TX_OVERHEAD), &info.sop, TX_OVERHEAD);
+	memcpy(skb_push(skb, TX_OVERHEAD), &info.tx_overhead, TX_OVERHEAD);
 
 	/* Write SPI TXQ header */
 	memcpy(skb_push(skb, spi_len), ax88796c_tx_cmd_buf, spi_len);

drivers/net/ethernet/asix/ax88796c_main.h

@@ -25,7 +25,7 @@
 #define AX88796C_PHY_REGDUMP_LEN	14
 #define AX88796C_PHY_ID			0x10
 
-#define TX_OVERHEAD			8
+#define TX_OVERHEAD     sizeof_field(struct tx_pkt_info, tx_overhead)
 #define TX_EOP_SIZE			4
 
 #define AX_MCAST_FILTER_SIZE		8
@@ -549,8 +549,10 @@ struct tx_eop_header {
 };
 
 struct tx_pkt_info {
-	struct tx_sop_header sop;
-	struct tx_segment_header seg;
+	struct_group(tx_overhead,
+		struct tx_sop_header sop;
+		struct tx_segment_header seg;
+	);
 	struct tx_eop_header eop;
 	u16 pkt_len;
 	u16 seq_num;