wifi: mac80211: ieee80211_i: Fix memory corruption bug in struct ieee80211_chanctx

Move the `struct ieee80211_chanctx_conf conf` to the end of
`struct ieee80211_chanctx` and fix a memory corruption bug
triggered e.g. in `hwsim_set_chanctx_magic()`: `radar_detected`
is being overwritten when `cp->magic = HWSIM_CHANCTX_MAGIC;`
See the function call sequence below:

drv_add_chanctx(... struct ieee80211_chanctx *ctx) ->
    local->ops->add_chanctx(&local->hw, &ctx->conf) ->
	mac80211_hwsim_add_chanctx(... struct ieee80211_chanctx_conf *ctx) ->
	    hwsim_set_chanctx_magic(ctx)

This also happens in a number of other drivers.

Also, add a code comment to try to prevent people from introducing
new members after `struct ieee80211_chanctx_conf conf`. Notice that
`struct ieee80211_chanctx_conf` is a flexible structure --a structure
that contains a flexible-array member, so it should always be at
the end of any other containing structures.

This change also fixes 50 of the following warnings:

net/mac80211/ieee80211_i.h:895:39: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Fixes: bca8bc0399 ("wifi: mac80211: handle ieee80211_radar_detected() for MLO")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/ZxwWPrncTeSi1UTq@kspp
[also refer to other drivers in commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/ieee80211_i.h

@@ -892,9 +892,10 @@ struct ieee80211_chanctx {
 	/* temporary data for search algorithm etc. */
 	struct ieee80211_chan_req req;
 
-	struct ieee80211_chanctx_conf conf;
-
 	bool radar_detected;
+
+	/* MUST be last - ends in a flexible-array member. */
+	struct ieee80211_chanctx_conf conf;
 };
 
 struct mac80211_qos_map {