Problem:

When the user specified foreign key name contains "_ibfk_", InnoDB wrongly
tries to rename it. 

Solution:

When a table is renamed, all its associated foreign keys will also be renamed,
only if the foreign key names are automatically generated.  If the foreign key
names are given by the user, even if it has _ibfk_ in it, it must not be
renamed.

rb#2935 approved by Jimmy, Krunal and Satya