Fonts: Support FONT_EXTRA_WORDS macros for font_6x8

Recently, in commit 6735b463 ("Fonts: Support FONT_EXTRA_WORDS macros
for built-in fonts"), we wrapped each of our built-in data buffers in a
`font_data` structure, in order to use the following macros on them, see
include/linux/font.h:

	#define REFCOUNT(fd)	(((int *)(fd))[-1])
	#define FNTSIZE(fd)	(((int *)(fd))[-2])
	#define FNTCHARCNT(fd)	(((int *)(fd))[-3])
	#define FNTSUM(fd)	(((int *)(fd))[-4])

	#define FONT_EXTRA_WORDS 4

Do the same thing to our new 6x8 font. For built-in fonts, currently we
only use FNTSIZE(). Since this is only a temporary solution for an
out-of-bounds issue in the framebuffer layer (see commit 5af08640
("fbcon: Fix global-out-of-bounds read in fbcon_get_font()")), all the
three other fields are intentionally set to zero in order to discourage
using these negative-indexing macros.

Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/926453876c92caac34cba8545716a491754d04d5.1603037079.git.yepeilin.cs@gmail.com