Correctly calculate data_end

 *     ,---------------------------  + head
 *    /          ,-----------------  + data
 *   /          /      ,-----------  + tail
 *  |          |      |            , + end
 *  |          |      |           |
 *  v          v      v           v
 *   -----------------------------------------------
 *  | headroom | data |  tailroom | skb_shared_info |
 *   -----------------------------------------------

According to above illustration of skb, data_end should be skb->head +
skb->tail.

Signed-off-by: Zhichuan Liang <[email protected]>

Author: jschwinger233

bpf/kprobe_pwru.c

@@ -140,14 +140,11 @@ filter_pcap_ebpf(void *_skb, void *__skb, void *___skb, void *data, void* data_e
 }
 
 static __always_inline bool
-filter_pcap(struct sk_buff *skb) {
-	BPF_CORE_READ(skb, head);
+filter_pcap(struct sk_buff *skb)
+{
 	void *skb_head = BPF_CORE_READ(skb, head);
-	u16 l3_off = BPF_CORE_READ(skb, network_header);
-	void *data = skb_head + l3_off;
-	u16 l4_off = BPF_CORE_READ(skb, transport_header);
-	u16 len = BPF_CORE_READ(skb, len);
-	void *data_end = skb_head + l4_off + len;
+	void *data = skb_head + BPF_CORE_READ(skb, network_header);
+	void *data_end = skb_head + BPF_CORE_READ(skb, tail);
 	return filter_pcap_ebpf((void *)skb, (void *)skb, (void *)skb, data, data_end);
 }