vt/fbcon: fix background color on line feed

Another addendum to commit c9e587ab
("vt: fix background color on line feed").

fbcon still was not doing the right thing (read: continued to do old
behavior).  fbcon_clear() seems to clear the new line (e.g.  where your new
prompt appears after doing echo -en "\e[42mfoo\n"), while scr_memsetw clears
the previous one only (where "foo" appears).  So just temporarily set the
video_erase_char to the scrl_erase_char so that fbcon_clear does the right
thing.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>