[media] vb2: regression fix: always set length field.

Commit dc77523c ensured that m.offset is
only set for the MMAP memory mode by calling __setup_offsets only for that
mode.

However, __setup_offsets also initializes the length fields, and that should
be done regardless of the memory mode. Because of that change the v4l2-ctl
test application fails for the USERPTR mode.

This fix creates a __setup_lengths function that sets the length, and
__setup_offsets just sets the offset and no longer touches the length.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>