Once the I/O completed we don't need the meta page anymore.  As the iounits
can live on for a long time this reduces memory pressure a bit.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Shaohua Li <shli@fb.com>
Signed-off-by: NeilBrown <neilb@suse.com>