wifi: mac80211: Set n_channels after allocating struct cfg80211_scan_request

Make sure that n_channels is set after allocating the
struct cfg80211_registered_device::int_scan_req member. Seen with
syzkaller:

UBSAN: array-index-out-of-bounds in net/mac80211/scan.c:1208:5
index 0 is out of range for type 'struct ieee80211_channel *[] __counted_by(n_channels)' (aka 'struct ieee80211_channel *[]')

This was missed in the initial conversions because I failed to locate
the allocation likely due to the "sizeof(void *)" not matching the
"channels" array type.

Reported-by: syzbot+4bcdddd48bb6f0be0da1@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/680fd171.050a0220.2b69d1.045e.GAE@google.com/
Fixes: e3eac9f ("wifi: cfg80211: Annotate struct cfg80211_scan_request with __counted_by")
Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/20250509184641.work.542-kees@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: kees

net/mac80211/main.c

@@ -1354,10 +1354,12 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 	hw->wiphy->software_iftypes |= BIT(NL80211_IFTYPE_MONITOR);
 
 
-	local->int_scan_req = kzalloc(sizeof(*local->int_scan_req) +
-				      sizeof(void *) * channels, GFP_KERNEL);
+	local->int_scan_req = kzalloc(struct_size(local->int_scan_req,
+						  channels, channels),
+				      GFP_KERNEL);
 	if (!local->int_scan_req)
 		return -ENOMEM;
+	local->int_scan_req->n_channels = channels;
 
 	eth_broadcast_addr(local->int_scan_req->bssid);