fix(ebpf): relax MySQL split header length check

[yinjiping]

When MySQL packet header is read separately, prev_count == 4 means the 4-byte MySQL header has already been cached and the current buffer starts from the payload.

The packet payload length in the MySQL header does not necessarily equal the current syscall read/write length. Requiring len == count may cause valid split packets to skip MySQL inference.

Remove this strict check and always parse seq from the cached header and command bytes from the current payload buffer in the prev_count == 4 path. The later full packet length validation is still kept for initial protocol confirmation.

This PR is for:

• Agent

Affected branches

• v6.6

[kylewanginchina]

[yinjiping]

#define INFER_BUF_MAX  32

/*
 * BPF Tail Calls context
 */
struct infer_data_s {
	__u32 len;
	char data[INFER_BUF_MAX * 2];
};

 infer_mysql_message(infer_buf, count,
					conn_info))

传递参数是 infer_buf， 他是固定大小的事先缓存内存块，mysql的推断用到的数据不会超过32字节, 推断的数据在9字节以内进行，因此不会出现越界情况，另外我count < 9检查防止脏数据。

     /*
	 * To prevent stale data from a previous map value remaining in
	 * the unused portion of `__infer_buf->data` when the current
	 * syscall provides fewer than 9 bytes of actual data.
	 */
	if (count < 9) {
		point_1 = point_2 = 0;
	}

我测试内核 4.14， 4.19， 5.15 正常

[yinjiping]

@claude review下这个PR