sd-journal: merge journal_file_next_entry_for_data() with generic_array_get_plus_one()

sd-journal: merge journal_file_next_entry_for_data() with generic_array_get_plus_one()

Because journal_file_next_entry_for_data() provides the first entry, while
journal_file_next_entry() actually provides the next entry of the input,
this also renames it to journal_file_move_to_entry_for_data().

Also, previously, on DIRECTION_UP the function did not fall back to the
'extra' entry when all entries linked in the chained array are broken.
This also fixes the issue, and now it fall back to the extra entry.