luci-base: form.js: reuse JSONMap data provider in GridSection modals

Reuse the same data provider as the parent JSONMap instance in order
to avoid inadvertently mangling the form data when saving the modal
edit dialog.

Fixes: bb9ede238a ("luci-base: form.js: reuse JSONMap data provider in GridSection modals")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>