net: iucv: 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/iucv/af_iucv.c

@@ -48,7 +48,7 @@ static struct iucv_interface *pr_iucv;
 static const u8 iprm_shutdown[8] =
 	{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01};
 
-#define TRGCLS_SIZE	(sizeof(((struct iucv_message *)0)->class))
+#define TRGCLS_SIZE	FIELD_SIZEOF(struct iucv_message, class)
 
 #define __iucv_sock_wait(sk, condition, timeo, ret)			\
 do {									\