Don't allow memory flush to write out segusage blocks.

Don't allow memory flush to write out segusage blocks.

We always write these after a checkpoint, and there is little to be
gained by writing them earlier, and doing so causes their
dirty status to be lost, which is bad.

They should be treated much like PinPending blocks, but they
are not PinPending as they are written later in the checkpoint.

Signed-off-by: NeilBrown <neilb@suse.de>