Adding a destination parameter on the end of any link with drupal_get_desitnation(). You can add it to the links render array using #options.

gistfile1.php

$vars['edit_reg'] = array(
  '#type' => 'link',
  '#href' => 'field-collection/field-event-instance/' . $registration_id . '/edit',
  '#title' => 'Edit',
  '#alt' => 'Edit registrations.',
  '#attributes' => array(
    'class' => 'button',
  ),
  '#options' => array('query' => drupal_get_destination()),
);