seq_buf_vprintf
seq_buf_vprintf(&last_cmd_status, fmt, ap);
int seq_buf_vprintf(struct seq_buf *s, const char *fmt, va_list args);
seq_buf_vprintf(s, fmt, args);
seq_buf_vprintf(&s->seq, fmt, args);
seq_buf_vprintf(&s->seq, fmt, ap);
ret = seq_buf_vprintf(s, fmt, ap);