zfs: Force an unmount ahead of destroy
zfs: Force an unmount ahead of destroy

It looks like in some cases we have a stopped container that still holds
some fs reference in the kernel, causing some problems at destroy time.
So let's do a lazy unmount ahead of time which seems to help the kernel
clean things up, making zfs succeed.

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

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