fbdev: sisfb: Fix strbuf array overflow

In the Linux kernel, the following vulnerability has been resolved:

fbdev: sisfb: Fix strbuf array overflow

The values of the variables xres and yres are placed in strbuf.

These variables are obtained from strbuf1.

The strbuf1 array contains digit characters

and a space if the array contains non-digit characters.

Then, when executing sprintf(strbuf, "%ux%ux8", xres, yres);

more than 16 bytes will be written to strbuf.

It is suggested to increase the size of the strbuf array to 24.

Found by Linux Verification Center (linuxtesting.org) with SVACE.