[stable] ipfrag: really prevent allocation on netns exit

Please pick this commit for the 4.9 and 4.14 stable branches:

commit f6f2a4a2eb92bc73671204198bb2f8ab53ff59fb Author: Paolo Abeni pabeni@redhat.com Date:   Fri Jul 6 12:30:20 2018 +0200

ipfrag: really prevent allocation on netns exit

It's not needed on earlier branches unless other commits are also applied.

Ben.