CVE-2026-74668: packet: use consistent hard_header_len in TX_RING send path

In the Linux kernel, the following vulnerability has been resolved:

packet: use consistent hardheaderlen in TXRING send path

tpacketsnd() reads dev->hardheaderlen independently for skb allocation and header construction in tpacketfillskb(). Concurrent netdevice reconfiguration can therefore make the reserved headroom smaller than the amount later pushed, or make copylen - hardheaderlen negative.

Snapshot hardheaderlen once before processing ring frames and use it for the frame limit, headroom allocation, copy length, and skb construction. Pass the snapshot to tpacketfillskb().

The separate SOCKDGRAM consistency problem between hardheaderlen and headerops->create is not addressed here.