media: media/v4l2-mem2mem.h: rename 'videobuf' to 'vb2'

It is confusing to refer to vb2 structures with 'videobuf', since
that typically is used to refer to the old videobuf version 1
framework.

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