Thanks for the feedback!

It hasn´t occurred again with 12.0-U3. 

Thank you

Can you confirm this still happens on latest 12.0-U3?

I have run the dtrace command as was suggested to Michael B.
The output is:

dtrace: description 'fbt::ifa_ref:entry ' matched 2 probes
CPU ID FUNCTION:NAME
0 71595 :tick-300s

3 45
7 151
4 152

I’m seeing this as well but in my instance I’m not convinced it’s a problem with freenas. 
I am building an additional NAS as a backup/long term storage for my primary freenas install. 
I have a HP DL380 server with a netapp qsfp interface card and a ds4246 shelf with iom6 controllers. 
I'm taking the opportunity to try out different hardware and experiment with stuff. It is very possible this setup has a hardware fault causing the panic and I’m in the process of trying to isolate the exact circumstances that trigger the panic.
So far I can be certain of the following.
No pools, only the boot pool exists, no panic.
Single pool on single disk attached directly to the servers built in controller with system data set on that pool, no panic.
Single pool on mirrored pair of 4TB disks in netapp shelf, system dataset in this pool, will panic at some point but can not determine if that is due to some specific system activity.
2 pools, one is on local disk with system dataset, other pool is mirrored pair of 4TB disks in netapp shelf, seems to keep working, no panic yet.

 Thanks.