net: core: Use FIELD_SIZEOF directly instead of reimplementing its function

FIELD_SIZEOF is defined as a macro to calculate the specified value. Therefore,
We prefer to use the macro rather than calculating its value.
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/flow_dissector.c

@@ -382,8 +382,8 @@ __skb_flow_dissect_gre(const struct sk_buff *skb,
 	offset += sizeof(struct gre_base_hdr);
 
 	if (hdr->flags & GRE_CSUM)
-		offset += sizeof(((struct gre_full_hdr *) 0)->csum) +
-			  sizeof(((struct gre_full_hdr *) 0)->reserved1);
+		offset += FIELD_SIZEOF(struct gre_full_hdr, csum) +
+			  FIELD_SIZEOF(struct gre_full_hdr, reserved1);
 
 	if (hdr->flags & GRE_KEY) {
 		const __be32 *keyid;
@@ -405,11 +405,11 @@ __skb_flow_dissect_gre(const struct sk_buff *skb,
 			else
 				key_keyid->keyid = *keyid & GRE_PPTP_KEY_MASK;
 		}
-		offset += sizeof(((struct gre_full_hdr *) 0)->key);
+		offset += FIELD_SIZEOF(struct gre_full_hdr, key);
 	}
 
 	if (hdr->flags & GRE_SEQ)
-		offset += sizeof(((struct pptp_gre_header *) 0)->seq);
+		offset += FIELD_SIZEOF(struct pptp_gre_header, seq);
 
 	if (gre_ver == 0) {
 		if (*p_proto == htons(ETH_P_TEB)) {
@@ -436,7 +436,7 @@ __skb_flow_dissect_gre(const struct sk_buff *skb,
 		u8 *ppp_hdr;
 
 		if (hdr->flags & GRE_ACK)
-			offset += sizeof(((struct pptp_gre_header *) 0)->ack);
+			offset += FIELD_SIZEOF(struct pptp_gre_header, ack);
 
 		ppp_hdr = __skb_header_pointer(skb, *p_nhoff + offset,
 					       sizeof(_ppp_hdr),