[PATCH] wireless: Use offsetof instead of custom macro.

[Pi-Hsun Shih]

Use offsetof to calculate offset of a field to take advantage of
compiler built-in version when possible, and avoid UBSAN warning when
compiling with Clang:

==================================================================
UBSAN: Undefined behaviour in net/wireless/wext-core.c:525:14
member access within null pointer of type 'struct iw_point'
CPU: 3 PID: 165 Comm: kworker/u16:3 Tainted: G S      W         4.19.23 #43
Workqueue: cfg80211 __cfg80211_scan_done [cfg80211]
Call trace:
 dump_backtrace+0x0/0x194
 show_stack+0x20/0x2c
 __dump_stack+0x20/0x28
 dump_stack+0x70/0x94
 ubsan_epilogue+0x14/0x44
 ubsan_type_mismatch_common+0xf4/0xfc
 __ubsan_handle_type_mismatch_v1+0x34/0x54
 wireless_send_event+0x3cc/0x470
 ___cfg80211_scan_done+0x13c/0x220 [cfg80211]
 __cfg80211_scan_done+0x28/0x34 [cfg80211]
 process_one_work+0x170/0x35c
 worker_thread+0x254/0x380
 kthread+0x13c/0x158
 ret_from_fork+0x10/0x18
===================================================================

Signed-off-by: Pi-Hsun Shih <pihsun@xxxxxxxxxxxx>
---
 include/uapi/linux/wireless.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/uapi/linux/wireless.h b/include/uapi/linux/wireless.h
index 86eca3208b6b..f259cca5cc2b 100644
--- a/include/uapi/linux/wireless.h
+++ b/include/uapi/linux/wireless.h
@@ -1090,8 +1090,7 @@ struct iw_event {
 /* iw_point events are special. First, the payload (extra data) come at
  * the end of the event, so they are bigger than IW_EV_POINT_LEN. Second,
  * we omit the pointer, so start at an offset. */
-#define IW_EV_POINT_OFF (((char *) &(((struct iw_point *) NULL)->length)) - \
-			  (char *) NULL)
+#define IW_EV_POINT_OFF offsetof(struct iw_point, length)
 #define IW_EV_POINT_LEN	(IW_EV_LCP_LEN + sizeof(struct iw_point) - \
 			 IW_EV_POINT_OFF)
 
-- 
2.21.0.392.gf8f6787159e-goog