can: dev: init_can_skb(): restore skb header initialization

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

can: dev: init_can_skb(): restore skb header initialization

Commit 9f10374bb024 ("can: remove private CAN skb headroom infrastructure")
removed the skb_reset_mac_header()/skb_reset_network_header()/
skb_reset_transport_header() calls from init_can_skb(). As a result, RX
skbs from alloc_can_skb() and friends again carry mac_header = 0xFFFF.
When such an skb reaches packet_rcv_spkt() (SOCK_PACKET), the push length
calculation overflows and triggers skb_under_panic -> kernel BUG -> full
machine panic.

The same issue was originally reported in 2014 on linux-can and fixed by
commit 969439016d2c ("can: add missing initialisations in CAN related
skbuffs"). packet_rcv_spkt() itself has never been hardened: only
packet_rcv() and tpacket_rcv() gained dev_has_header() checks in
commit d549699048b4 ("net/packet: fix packet receive on L3 devices
without visible hard header").