wifi: iwlwifi: mvm: Fix __counted_by usage in cfg80211_wowlan_nd_*

wifi: iwlwifi: mvm: Fix __counted_by usage in cfg80211_wowlan_nd_*

Both struct cfg80211_wowlan_nd_match and struct cfg80211_wowlan_nd_info
pre-allocate space for channels and matches, but then may end up using
fewer that the full allocation. Shrink the associated counter
(n_channels and n_matches) after counting the results. This avoids
compile-time (and run-time) warnings from __counted_by. (The counter
member needs to be updated _before_ accessing the array index.)

Seen with coming GCC 15:

drivers/net/wireless/intel/iwlwifi/mvm/d3.c: In function 'iwl_mvm_query_set_freqs':
drivers/net/wireless/intel/iwlwifi/mvm/d3.c:2877:66: warning: operation on 'match->n_channels' may be undefined [-Wsequence-point]
 2877 |                                 match->channels[match->n_channels++] =
      |                                                 ~~~~~~~~~~~~~~~~~^~
drivers/net/wireless/intel/iwlwifi/mvm/d3.c:2885:66: warning: operation on 'match->n_channels' may be undefined [-Wsequence-point]
 2885 |                                 match->channels[match->n_channels++] =
      |                                                 ~~~~~~~~~~~~~~~~~^~
drivers/net/wireless/intel/iwlwifi/mvm/d3.c: In function 'iwl_mvm_query_netdetect_reasons':
drivers/net/wireless/intel/iwlwifi/mvm/d3.c:2982:58: warning: operation on 'net_detect->n_matches' may be undefined [-Wsequence-point]
 2982 |                 net_detect->matches[net_detect->n_matches++] = match;
      |                                     ~~~~~~~~~~~~~~~~~~~~~^~

Cc: stable@vger.kernel.org
Fixes: aa4ec06c455d ("wifi: cfg80211: use __counted_by where appropriate")
Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/20240619211233.work.355-kees@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>