A pending packet uses a cookie as an unique key, but it can be duplicated
because it didn't use atomic operators.

And also, a pending packet can be null in hwsim_tx_info_frame_received_nl
due to race condition with mac80211_hwsim_stop.

For this,
 * Use an atomic type and operator for a cookie
 * Add a lock around the loop for pending packets
Signed-off-by: NJeongik Cha <jeongik@google.com>
Link: https://lore.kernel.org/r/20220704084354.3556326-1-jeongik@google.comSigned-off-by: NJohannes Berg <johannes.berg@intel.com>