pnfs: use lwb as layoutcommit length

commit 3557c6c3 upstream.

Using NFS4_MAX_UINT64 will break current protocol.

[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>