LU-3125 layout: allow stripeless layouts swap

* allow stripeless layouts swap
* `lfs swap_layouts` should open with O_LOV_DELAY_CREATE

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I3947005d1060ee9be189a9c2adbab61064a8e6f0
Reviewed-on: http://review.whamcloud.com/5998
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>