fbdev: improve fb_mmap bounds checks

Improve fb_mmap bounds checks in gbefb, smscufx, udlfb and vfb drivers to
prevent possible uint overflows.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Steve Glendinning <steve.glendinning@smsc.com>
Cc: Bernie Thompson <bernie@plugable.com>

diff --cc drivers/video/gbefb.c
Simple merge

diff --cc drivers/video/smscufx.c
Simple merge

diff --cc drivers/video/udlfb.c
Simple merge

diff --cc drivers/video/vfb.c
Simple merge