The size of the vma can be larger than the size of the backing buffer.
Use the buffer size over vma size to prevent exposing extra memory
to userspace.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>