video: fbdev: i740fb: Check the argument of i740_calc_vclk()

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

video: fbdev: i740fb: Check the argument of i740_calc_vclk()

Since the user can control the arguments of the ioctl() from the user

space, under special arguments that may result in a divide-by-zero bug.

If the user provides an improper 'pixclock' value that makes the argumet

of i740_calc_vclk() less than 'I740_RFREQ_FIX', it will cause a

divide-by-zero bug in:

drivers/video/fbdev/i740fb.c:353 p_best = min(15, ilog2(I740_MAX_VCO_FREQ / (freq / I740_RFREQ_FIX)));

The following log can reveal it:

divide error: 0000 [#1] PREEMPT SMP KASAN PTI

RIP: 0010:i740_calc_vclk drivers/video/fbdev/i740fb.c:353 [inline]

RIP: 0010:i740fb_decode_var drivers/video/fbdev/i740fb.c:646 [inline]

RIP: 0010:i740fb_set_par+0x163f/0x3b70 drivers/video/fbdev/i740fb.c:742

Call Trace:

fb_set_var+0x604/0xeb0 drivers/video/fbdev/core/fbmem.c:1034

do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1110

fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1189

Fix this by checking the argument of i740_calc_vclk() first.