staging/lustre/lnet: portal spreading rotor should be unsigned

Portal spreading rotor should be unsigned, otherwise lnet may get
negative CPT number and access invalid addresses.

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-on: http://review.whamcloud.com/11936
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5639


Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Doug Oucharek <doug.s.oucharek@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>