UPSTREAM: fbcon: Prevent that screen size is smaller than font size

commit e64242ca upstream.

We need to prevent that users configure a screen size which is smaller than the
currently selected font size. Otherwise rendering chars on the screen will
access memory outside the graphics memory region.

This patch adds a new function fbcon_modechange_possible() which
implements this check and which later may be extended with other checks
if necessary.  The new function is called from the FBIOPUT_VSCREENINFO
ioctl handler in fbmem.c, which will return -EINVAL if userspace asked
for a too small screen size.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Bug: b8121282
Bug: 255071426
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I6ac4cce2aeea4dcca222ea2b395cc2baa1008894
(cherry picked from commit 2823b388)