vdo: extend volume and pool without flush

When the volume size is extended, there is no need to flush
IO operations (nothing can be targeting new space yet).
VDO target is supported as target that can safely work with
this condition.

Such support is also needed, when extending VDOPOOL size
while the pool is reaching its capacity - since this allows
to continue working without reaching 'out-of-space' condition
due to flushing of all in flight IO.

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