net: test for not too small csum_start in virtio_net_hdr_to_skb()

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

net: test for not too small csum_start in virtio_net_hdr_to_skb()

syzbot was able to trigger this warning [1], after injecting a
malicious packet through af_packet, setting skb->csum_start and thus
the transport header to an incorrect value.

We can at least make sure the transport header is after
the end of the network header (with a estimated minimal size).

[1]
[ 67.873027] skb len=4096 headroom=16 headlen=14 tailroom=0
mac=(-1,-1) mac_len=0 net=(16,-6) trans=10
shinfo(txflags=0 nr_frags=1 gso(size=0 type=0 segs=0))
csum(0xa start=10 offset=0 ip_summed=3 complete_sw=0 valid=0 level=0)
hash(0x0 sw=0 l4=0) proto=0x0800 pkttype=0 iif=0
priority=0x0 mark=0x0 alloc_cpu=10 vlan_all=0x0
encapsulation=0 inner(proto=0x0000, mac=0, net=0, trans=0)
[ 67.877172] dev name=veth0_vlan feat=0x000061164fdd09e9
[ 67.877764] sk family=17 type=3 proto=0
[ 67.878279] skb linear: 00000000: 00 00 10 00 00 00 00 00 0f 00 00 00 08 00
[ 67.879128] skb frag: 00000000: 0e 00 07 00 00 00 28 00 08 80 1c 00 04 00 00 02
[ 67.879877] skb frag: 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.880647] skb frag: 00000020: 00 00 02 00 00 00 08 00 1b 00 00 00 00 00 00 00
[ 67.881156] skb frag: 00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.881753] skb frag: 00000040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.882173] skb frag: 00000050: 00 00 00 00 00 00 00…