LU-2017 mdc: add layout swap between 2 objects

LU-2017 mdc: add layout swap between 2 objects

This patch adds the client and MDT code to swap layouts
between 2 files:
- a lfs command
- a llapi call, based on a new ioctl
- the mdc/mdt codes for the ioctl (with layout locking)

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I7e710a1ab3ca38e8b26582e49d08e9943aa445cd
Reviewed-on: http://review.whamcloud.com/4507
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>