V4L/DVB: videobuf: rename videobuf_alloc to videobuf_alloc_vb

These functions allocate videobuf_buffer structures only. Renaming in order
to prevent confusion with functions allocating actual video buffer memory.

Rename the functions in videobuf-core.h videobuf-dma-sg.c as well.

Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>