Revert "[media] v4l: vb2-memops: use vma slab when vma allocation"

Please revert this patch: vm_area_cachep is not exported, so you cannot use
this in a module.

This reverts commit 5ed1c328.

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