[media] videobuf2-vmalloc: fix sparse warning

Fix this warning:

drivers/media/v4l2-core/videobuf2-vmalloc.c:98:28: warning: incorrect type in assignment (different address spaces)
drivers/media/v4l2-core/videobuf2-vmalloc.c:158:28: warning: incorrect type in argument 1 (different address spaces)

The warning is correct, but we have no other choice here to forcibly cast.
At least it is now explicit that such a cast is needed.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>