Revert commit https://github.com/torvalds/linux/commit/a0ade8404c3bc2bf2631cb0f20d372eed22d9d96

The change caused API regression by turning fixed size struct to
flexible size struct. It was an unintentional change, broke `udp2raw`:
    https://github.com/NixOS/nixpkgs/pull/252587#issuecomment-1744427473
--- a/include/uapi/linux/if_packet.h
+++ b/include/uapi/linux/if_packet.h
@@ -18,11 +18,7 @@ struct sockaddr_ll {
 	unsigned short	sll_hatype;
 	unsigned char	sll_pkttype;
 	unsigned char	sll_halen;
-	union {
-		unsigned char	sll_addr[8];
-		/* Actual length is in sll_halen. */
-		__DECLARE_FLEX_ARRAY(unsigned char, sll_addr_flex);
-	};
+	unsigned char	sll_addr[8];
 };
 
 /* Packet types */
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -3607,7 +3607,7 @@ static int packet_getname(struct socket *sock, struct sockaddr *uaddr,
 	if (dev) {
 		sll->sll_hatype = dev->type;
 		sll->sll_halen = dev->addr_len;
-		memcpy(sll->sll_addr_flex, dev->dev_addr, dev->addr_len);
+		memcpy(sll->sll_addr, dev->dev_addr, dev->addr_len);
 	} else {
 		sll->sll_hatype = 0;	/* Bad: we have no ARPHRD_UNSPEC */
 		sll->sll_halen = 0;