lxd/storage/zfs: Tweak destroy logic
lxd/storage/zfs: Tweak destroy logic

This attempts to workaround some reported issues around dataset deletion
by first attempting a clean umount, then a lazy umount and also adding a
1s delay in the case where we unmounted something.

This path is however unlikely to ever be taken on a normal system as LXD
should have unmounted the container before hitting that point.

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

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