media: vb2: videobuf -> videobuf2

It is confusing to use the term 'videobuf' or 'video-buf' since that
usually refers to the old videobuf version 1 framework. Rename to
'videobuf2' or vb2.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>