fbdev: bound mode sysfs output to the sysfs buffer

In the Linux kernel, the following vulnerability has been resolved: fbdev: bound mode sysfs output to the sysfs buffer mode_string() uses snprintf() which can return a value larger than the remaining buffer space. show_modes() accumulates the return value into i without checking whether i has reached PAGE_SIZE, causing the offset to advance past the sysfs buffer if the modelist is long enough. Add a size parameter to mode_string() and use scnprintf() to return only the bytes actually written. Add an early return when offset already exceeds the buffer. In show_modes(), stop accumulating once the buffer is full.