MDEV-16429: Assertion `!table || (!table->read_set || bitmap_is_set(table->read_set, field_index))' fails upon attempt to update virtual column on partitioned versioned table

When using buffered sort in `UPDATE`, keyread is used. In this case,
`TABLE::update_virtual_field` should be aborted, but it actually isn't,
because it is called not with a top-level handler, but with the one that
is actually going to access the disk. Here the problemm is issued with
partitioning, so the solution is to recursively mark for keyread all the
underlying partition handlers.

* ha_partition: update keyread state for child partitions

Closes #800