pnfs/blocklayout: implement the return_range method

This allows removing extents from the extent tree especially on truncate
operations, and thus fixing reads from truncated and re-extended that
previously returned stale data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>