staging/lustre: constify uuid argument of obd_uuid2str

staging/lustre: constify uuid argument of obd_uuid2str

This is only part of the original Lustre tree commit.
Main part of the original commit changes server code and is
unneeded at client side.

Lustre-change: http://review.whamcloud.com/6534
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3342
Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>