fbcon: prevent user font height or width change from causing potential out-of-bounds access

Add a check to fbcon_resize() to ensure that a possible change to user font
height or user font width will not allow a font data out-of-bounds access.
NOTE: must use original charcount in calculation as font charcount can
change and cannot be used to determine the font data allocated size.

Signed-off-by: George Kennedy <george.kennedy@oracle.com>
Cc: stable <stable@vger.kernel.org>
Reported-by:  <syzbot+38a3699c7eaf165b97a6@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/1596213192-6635-1-git-send-email-george.kennedy@oracle.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>