Bug#49836 reports that the geometry type does not work
with WL#5151 applied.

The GEOMETRY type inherits the blob comparison function,
which read the pack length from the metadata. The GEOMETRY
type does not fill in the metadata with anything sensible,
so it is always zero, meaning that the pack length for the
source type is considered zero, rendering it always "smaller"
than the target type which has pack length 4 (without pointer).

This patch fixes the problem by defining
Field_geom::pack_length_from_metadata() to always use the
same as Field_geom::row_pack_length().