Fix calculation of attribute_data_offset

[lukasdreyer]

Describe your changes here:

This PR fixes the multiple attributes test for at least 8 procs.
The attribute data offset update did not include the number of ghost attributes, which it does now.

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[lukasdreyer]

@jmark: Regarding our discussion, whether ghost_tree_attributes can be handled in the same way as tree_attributes:
I do not think they can, as the local order of ghosts when creating the cmesh from stash is not given by the global order of treeids, but by the order in which they are seen by facejoins. Therefore, different to tree attributes, the ghosts are not visited in the same order that they are saved in the char*, therefore the "optimization" done in t8_cmesh_trees_get_attribute, where the next attribute info offset is already set, cannot be done for the ghost attributes.