I created a new c5d.large instance on AWS, and installed 4.4.0-177-generic from -updates. I ran through the reproduction steps, and the issue reproduces.
I then created a new c5d.large instance, enabled -proposed and installed 4.4.0-178-generic. I again ran through the reproduction steps, and this time, the final mount succeeds, there is no segmentation fault and dmesg is clean of any errors.
I created a new c5d.large instance on AWS, and installed 4.4.0-177-generic from -updates. I ran through the reproduction steps, and the issue reproduces.
I then created a new c5d.large instance, enabled -proposed and installed 4.4.0-178-generic. I again ran through the reproduction steps, and this time, the final mount succeeds, there is no segmentation fault and dmesg is clean of any errors.
I am happy to mark this as verified.