unable to delete iscsi file extents

I now have a problem with freenas that when i delete an iscsi target the extent gets left behind.  This is resulting in wasted space with files that seem undeleteable.  I have tried to remove the extent file via the shell but get errors there too. 

There's a previously entered bug entry in redmine for this:

https://redmine.ixsystems.com/issues/76278 

This appears tob e naother middleware issue...and i do not wish to run 12 beta on a production mahcine:

https://jira.ixsystems.com/browse/NAS-104191 

Can we get this fix backported to 11.3u5?