This bug ensures that a live downgrade from an InnoDB 5.6 with WL5756 and
a database created with innodb-page-size=8k or 4k will make a version 5.5
installation politely refuse to start. Instead of crashing or giving some
indication about a corrupted database, it will indicate the page size
difference. 

This patch takes only that part of the Wl5756 patch that is needed to
protect against opening a tablespace that is stamped with a different
page size.

It also contains the change in dict_index_find_on_id_low() just in case
a database with another page size is created by recompiling a pre-WL5756
InnoDB.