wifi: mac80211: Use flexible array in struct ieee80211_tim_ie

Currently struct ieee80211_tim_ie defines:
	u8 virtual_map[1];

Per the guidance in [1] change this to be a flexible array.

Per the discussion in [2] wrap the virtual_map in a union with a u8
item in order to preserve the existing expectation that the
virtual_map must contain at least one octet (at least when used in a
non-S1G PPDU). This means that no driver changes are required.

[1] https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays
[2] https://lore.kernel.org/linux-wireless/202308301529.AC90A9EF98@keescook/Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230831-ieee80211_tim_ie-v3-2-e10ff584ab5d@quicinc.com
[add wifi prefix]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/linux/ieee80211.h

@@ -951,17 +951,24 @@ struct ieee80211_wide_bw_chansw_ie {
  * @dtim_count: DTIM Count
  * @dtim_period: DTIM Period
  * @bitmap_ctrl: Bitmap Control
+ * @required_octet: "Syntatic sugar" to force the struct size to the
+ *                  minimum valid size when carried in a non-S1G PPDU
  * @virtual_map: Partial Virtual Bitmap
  *
  * This structure represents the payload of the "TIM element" as
- * described in IEEE Std 802.11-2020 section 9.4.2.5.
+ * described in IEEE Std 802.11-2020 section 9.4.2.5. Note that this
+ * definition is only applicable when the element is carried in a
+ * non-S1G PPDU. When the TIM is carried in an S1G PPDU, the Bitmap
+ * Control and Partial Virtual Bitmap may not be present.
  */
 struct ieee80211_tim_ie {
 	u8 dtim_count;
 	u8 dtim_period;
 	u8 bitmap_ctrl;
-	/* variable size: 1 - 251 bytes */
-	u8 virtual_map[1];
+	union {
+		u8 required_octet;
+		DECLARE_FLEX_ARRAY(u8, virtual_map);
+	};
 } __packed;
 
 /**