wifi: mac80211_hwsim: check length for virtio packets

An invalid packet with a length shorter than the specified length in the
netlink header can lead to use-after-frees and slab-out-of-bounds in the
processing of the netlink attributes, such as the following:

  BUG: KASAN: slab-out-of-bounds in __nla_validate_parse+0x1258/0x2010
  Read of size 2 at addr ffff88800ac7952c by task kworker/0:1/12

  Workqueue: events hwsim_virtio_rx_work
  Call Trace:
   <TASK>
   dump_stack_lvl+0x45/0x5d
   print_report.cold+0x5e/0x5e5
   kasan_report+0xb1/0x1c0
   __nla_validate_parse+0x1258/0x2010
   __nla_parse+0x22/0x30
   hwsim_virtio_handle_cmd.isra.0+0x13f/0x2d0
   hwsim_virtio_rx_work+0x1b2/0x370
   process_one_work+0x8df/0x1530
   worker_thread+0x575/0x11a0
   kthread+0x29d/0x340
   ret_from_fork+0x22/0x30
 </TASK>

Discarding packets with an invalid length solves this.
Therefore, skb->len must be set at reception.

Change-Id: Ieaeb9a4c62d3beede274881a7c2722c6c6f477b6
Signed-off-by: Soenke Huster <soenke.huster@eknoes.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>