net/packet: guard the ll header push in packet_rcv_spkt()

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

net/packet: guard the ll header push in packet_rcv_spkt()

packet_rcv_spkt() restores the link layer header with

skb_push(skb, skb->data - skb_mac_header(skb));

That subtraction is only meaningful when the device actually has a link
layer header. packet_rcv() and tpacket_rcv() both wrap it in
dev_has_header(), which is also the predicate the block comment at the
top of the file states the restore in terms of; packet_rcv_spkt() does
not. commit d549699048b4 ("net/packet: fix packet receive on L3
devices without visible hard header") introduced the helper and changed
the two call sites, and this one stayed behind.

A device without a visible ll header can leave skb->mac_header at the
0xFFFF sentinel that __alloc_skb() initialises it to. A CAN skb does:
init_can_skb() sets pkt_type and ip_summed but does not reset the
headers, and commit 9f10374bb024 ("can: remove private CAN skb
headroom infrastructure") dropped the skb_reset_*_header() calls that
used to be there. skb_mac_header() is then 0xFFFF, the length becomes
a large negative number and skb_push() reports it through
skb_under_panic() -- from softirq context, so it is a full system panic
even with panic_on_oops=0:

skbuff: skb_under_panic: text:ffffffff8bd21bc1 len:-65455 put:-65471 head:... data:... tail:0x50 end:0x180 dev:can0
kernel BUG at net/core/skbuff.c:214!
RIP: 0010:skb_panic+0x50/0x60
Call Trace:
<IRQ>
skb_push+0x38/0x40
packet_rcv_spkt+0xe1/0x170
__netif_receive_skb_core.constprop.0+0x7e8/0xd30
...
Kernel panic - not syncing: Fatal exception in interrupt

The socket type is reachable: packet_create() accepts SOCK_PACKET
alongside SOCK_RAW and SOCK_DGRAM behind the same CAP_NET_RAW check,
and neither the socket length nor a capability check keeps it away
from a CAN interface.

The missing skb_reset_*_header() calls in init_can_skb() are a
regression in their own right and are being fixed separately, but a
packet socket should not turn a link layer that did not initialise its
mac header into a kernel panic. Guard the push the way the other two
receive paths do.

Tested on v7.3-rc5 under QEMU with a slcan device on a pty, which is
the driver RX path: vcan does not reproduce it, because can_send()
resets the headers on the way out. One SOCK_PACKET socket bound to
can0 and one frame written into the line discipline panics an
unpatched kernel with the trace above; the same image with this patch
prints no panic and powers off normally. Both kernels are this tree,
defconfig plus CONFIG_CAN_SLCAN=y, differing only in this hunk.