Btrfs: dev-replace: skip prealloc extents when copy nocow pages

It doens't make sense to process prealloc extents as pages will be
filled with zero when reading prealloc extents.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>