Return fail if lv_deactivate did not removed device from kernel.

Return fail if lv_deactivate did not removed device from kernel.

lv_deactivate now returns always success, because tree deactivation
functions (see dm_tree_deactivate_children) always returns success.

Because code should return failure in lv_deactivate at least,
fix it by checking for device existence after real deactivation call.

(After discussion this was prefered solution to dm tree function rewrite
which affects snapshots and mirrors.)