LinuxKPI: 802.11: set IEEE80211_MIN_ACTION_SIZE according to struct

IEEE80211_MIN_ACTION_SIZE came up in the mt76 vendor subtree merge
as one of the non-mechanical changes which made me look.
Rather than (incorrectly) hand counting the offset from the beginning
of a frame, use the offset from the beginning of struct ieee80211_mgmt.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days

sys/compat/linuxkpi/common/include/linux/ieee80211.h

@@ -100,8 +100,6 @@ struct ieee80211_mmie_16 {
 
 #define	IEEE80211_MAX_RTS_THRESHOLD		2346	/* net80211::IEEE80211_RTS_MAX */
 
-#define	IEEE80211_MIN_ACTION_SIZE		23	/* ? */
-
 /* Wi-Fi Peer-to-Peer (P2P) Technical Specification */
 #define	IEEE80211_P2P_OPPPS_CTWINDOW_MASK	0x7f
 #define	IEEE80211_P2P_OPPPS_ENABLE_BIT		BIT(7)
@@ -622,6 +620,8 @@ struct ieee80211_mgmt {
 	} u;
 } __packed __aligned(2);
 
+#define	IEEE80211_MIN_ACTION_SIZE	offsetof(struct ieee80211_mgmt, u.action.u)
+
 struct ieee80211_cts {		/* net80211::ieee80211_frame_cts */
         __le16		frame_control;
         __le16		duration;