torture: Fix typo suppressing CPU-hotplug statistics

The torture status line contains a series of values preceded by "onoff:".
The last value in that line, the one preceding the "HZ=" string, is
always zero.  The reason that it is always zero is that torture_offline()
was incrementing the sum_offl pointer instead of the value that this
pointer referenced.  This commit therefore makes this increment operate
on the statistic rather than the pointer to the statistic.

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