Bluetooth: Avoid a couple dozen -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the __struct_group() helper to fix 31 instances of the following
type of warnings:

30 net/bluetooth/mgmt_config.c:16:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
1 net/bluetooth/mgmt_config.c:22:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

Author: GustavoARSilva

include/net/bluetooth/mgmt.h

@@ -53,10 +53,15 @@ struct mgmt_hdr {
 } __packed;
 
 struct mgmt_tlv {
-	__le16 type;
-	__u8   length;
+	/* New members MUST be added within the __struct_group() macro below. */
+	__struct_group(mgmt_tlv_hdr, __hdr, __packed,
+		__le16 type;
+		__u8   length;
+	);
 	__u8   value[];
 } __packed;
+static_assert(offsetof(struct mgmt_tlv, value) == sizeof(struct mgmt_tlv_hdr),
+	      "struct member likely outside of __struct_group()");
 
 struct mgmt_addr_info {
 	bdaddr_t	bdaddr;

net/bluetooth/mgmt_config.c

@@ -13,13 +13,13 @@
 
 #define HDEV_PARAM_U16(_param_name_) \
 	struct {\
-		struct mgmt_tlv entry; \
+		struct mgmt_tlv_hdr entry; \
 		__le16 value; \
 	} __packed _param_name_
 
 #define HDEV_PARAM_U8(_param_name_) \
 	struct {\
-		struct mgmt_tlv entry; \
+		struct mgmt_tlv_hdr entry; \
 		__u8 value; \
 	} __packed _param_name_