rcutorture: Handle different mpstat versions

The mpstat command recently added the %gnice column, which messes up
the cpu2use.sh script's idle-CPU calculations.  This commit therefore
uses $NF instead of $12 to select the last (%idle) column.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
1 file changed