In the function arkfb_set_par(), the value of 'screen_size' is
calculated by the user input. If the user provides the improper value,
the value of 'screen_size' may larger than 'info->screen_size', which
may cause the following bug:

[  659.399066] BUG: unable to handle page fault for address: ffffc90003000000
[  659.399077] #PF: supervisor write access in kernel mode
[  659.399079] #PF: error_code(0x0002) - not-present page
[  659.399094] RIP: 0010:memset_orig+0x33/0xb0
[  659.399116] Call Trace:
[  659.399122]  arkfb_set_par+0x143f/0x24c0
[  659.399130]  fb_set_var+0x604/0xeb0
[  659.399161]  do_fb_ioctl+0x234/0x670
[  659.399189]  fb_ioctl+0xdd/0x130

Fix the this by checking the value of 'screen_size' before memset_io().

Fixes: 681e1473 ("arkfb: new framebuffer driver for ARK Logic cards")
Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>