vt: skip screen update for DEC alignment test on backgroup consoles

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

vt: skip screen update for DEC alignment test on backgroup consoles

[BUG]
Recently, we encountered a KASAN warning as follows:

BUG: KASAN: slab-out-of-bounds in fb_pad_aligned_buffer+0x11f/0x140
Read of size 1 at addr ff1100015fd9f6a4 by task tty_fbcon_oob/1239
CPU: 7 UID: 0 PID: 1239 Comm: tty_fbcon_oob Not tainted 7.3.0-rc1 #100 PREEMPT(full)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014
Call Trace:
<TASK>
...
kasan_report+0xf0/0x120
fb_pad_aligned_buffer+0x11f/0x140
ccw_putcs+0x86c/0xa80
fbcon_putcs+0x338/0x410
do_update_region+0x21d/0x450
do_con_write+0x1e0e/0x4880
con_write+0x13/0x80
n_tty_write+0x374/0x1010
file_tty_write.isra.0+0x404/0x7a0
...

reproduce:
1) open /dev/tty0, set a KDFONTOP ioctl with op.op = KD_FONT_OP_SET,
op.width = 8 and op.height = 16 (visible VC1)
2) open /dev/tty1, set a KDFONTOP ioctl with op.op = KD_FONT_OP_SET,
op.width = 28 and op.height = 24 (invisible VC2)
3) echo 3 > /sys/devices/virtual/graphics/fbcon/rotate_all
4) write EShash8(esc hash8) to tty1

[CAUSE]
All VCs render to the framebuffer. setfont only modifies the target VC's
font without resizing the framebuffer backing buffer (par->rotated.buf);
the buffer is only resized for the visible VC
(fbcon_do_set_font -> ... -> vc_do_resize -> update_screen). This relies on
the con_should_update() check performed before every update_region().

However, the ESC # 8 path (do_con_trol -> do_update_region) omits the
con_should_update() check. After changing the font size of an invisible VC,
do_update_region() fills using the new font size against a buffer that was
never resized, causing an out-of-bounds access.

[FIX]
Only push the update when the console is visible and not blanked, add
the con_should_update() check in the do_con_trol() like every other call
site of do_update_region() (update_region(), invert_screen(), ...).