LU-2809 llite: Do not return layout_gen for getxattr

LU-2809 llite: Do not return layout_gen for getxattr

The problem is that layout_gen and stripe_offset are sharing the
same field in lov_user_md{}. Layout gen would be wrongly interpreted
as stripe_offset when a backup is restored.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: If2c120bf861eeffd2db5f92e1d23cb1b9a2f5c63
Reviewed-on: http://review.whamcloud.com/5664
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John Hammond <johnlockwoodhammond@gmail.com>