pnfs: save layoutcommit lwb at layout header

commit acff5880 upstream.

No need to save it for every lseg.

[Needed in v3.0]
Signed-off-by: Peng Tao <peng_tao@emc.com>
Signed-off-by: Jim Rees <rees@umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>