staging: r8188eu: calculate the delba length

Use offsetofend to calculate the length of the delba message.

We are now calculating all message lengths based on the offset of their
last field. All intermediate updates of pattrib->pktlen can be removed.
Tested-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220602193726.280922-10-martin@kaiser.cxSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/r8188eu/core/rtw_mlme_ext.c

@@ -5401,10 +5401,7 @@ void issue_action_BA(struct adapter *padapter, unsigned char *raddr, unsigned ch
 	mgmt->seq_ctrl = cpu_to_le16(pmlmeext->mgnt_seq);
 	pmlmeext->mgnt_seq++;
 
-	pattrib->pktlen = sizeof(struct ieee80211_hdr_3addr);
-
 	mgmt->u.action.category = WLAN_CATEGORY_BACK;
-	pattrib->pktlen++;
 
 	switch (action) {
 	case WLAN_ACTION_ADDBA_REQ:
@@ -5448,14 +5445,12 @@ void issue_action_BA(struct adapter *padapter, unsigned char *raddr, unsigned ch
 		break;
 	case WLAN_ACTION_DELBA:
 		mgmt->u.action.u.delba.action_code = WLAN_ACTION_DELBA;
-		pattrib->pktlen++;
 		mgmt->u.action.u.delba.params = cpu_to_le16((status & 0x1F) << 3);
 		params = u16_encode_bits((status & 0x1), IEEE80211_DELBA_PARAM_INITIATOR_MASK);
 		params |= u16_encode_bits((status >> 1) & 0xF, IEEE80211_DELBA_PARAM_TID_MASK);
 		mgmt->u.action.u.delba.params = cpu_to_le16(params);
-		pattrib->pktlen += 2;
 		mgmt->u.action.u.delba.reason_code = cpu_to_le16(WLAN_STATUS_REQUEST_DECLINED);
-		pattrib->pktlen += 2;
+		pattrib->pktlen = offsetofend(struct ieee80211_mgmt, u.action.u.delba.reason_code);
 		break;
 	default:
 		break;