pnfsblock: merge extents

Replace a stub, so that extents underlying the layouts are properly
added, merged, or ignored as necessary.

Signed-off-by: Fred Isaman <iisaman@citi.umich.edu>
[pnfsblock: delete the new node before put it]
Signed-off-by: Mingyang Guo <guomingyang@nrchpc.ac.cn>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Peng Tao <peng_tao@emc.com>
Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: Jim Rees <rees@umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>