Resolve some missing-field-initializers warnings by using
designated initialization.

[hch: W=2 with modern gcc warns about this.  Pretty pointless to me, but
 I'd prefer to keep us warning free]
Signed-off-by: NMark Rustad <mark.d.rustad@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>