LU-2445 utils: lfs migrate support

LU-2445 utils: lfs migrate support

Add a new command migrate to lfs utility.
migrate takes sames args as setstripe and a --block option
It:
- create a volatile file based on new stripe information
- copy file data to this volatile file
- swap the 2 layouts
- if --block is set, take the grouplock to block other access
  during copy

So after migrate the file is restriped (mtime, atime are not
changed).
lfs_migrate is also updated to use the new call and
keeps the rsync mode as a fallback.

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: I96bafb0be9bc273295c4c900c65b4028864fcbaa
Reviewed-on: http://review.whamcloud.com/5620
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>