wifi: mac80211_hwsim: clamp virtio RX length before skb_put

wifi: mac80211_hwsim: clamp virtio RX length before skb_put

hwsim_virtio_rx_work() passes the virtqueue used-ring length reported by
the device straight to skb_put() on a fixed-size receive skb. A backend
reporting a length larger than the skb tailroom drives skb_put() past the
buffer end and hits skb_over_panic() -- a host-triggerable guest panic
(denial of service).

Clamp the length to the skb's available room before skb_put(). A
conforming device never reports more than the posted buffer size, so valid
frames are unaffected; a truncated over-report then fails the
length/header checks in hwsim_virtio_handle_cmd() and is dropped, so
truncating rather than dropping here cannot be turned into a parsing
problem.

Fixes: 5d44fe7c9808 ("mac80211_hwsim: add frame transmission support over virtio")
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Link: https://patch.msgid.link/20260620-b4-disp-474bee37-v1-1-1a4d37f3e2d4@proton.me
Signed-off-by: Johannes Berg <johannes.berg@intel.com>