Merge r2166:2168 from trunk/:

Bug#32440:

Put information about the free space in a tablespace in
INFORMATION_SCHEMA.TABLES.DATA_FREE. This information was previously
available in INFORMATION_SCHEMA.TABLES.TABLE_COMMENT, but MySQL has
removed it from there recently.

The stored value is in kilobytes.

This can be considered as a permanent workaround to
http://bugs.mysql.com/32440. "Workaround" becasue that bug is about the
data missing from TABLE_COMMENT and this is actually not solved.