wifi: mac80211: Use flexible array in struct ieee80211_tim_ie

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>