torture: Don't try to offline the last CPU

If there is only one online CPU, it doesn't make sense to try to offline
it, as any such attempt is guaranteed to fail.  This commit therefore
check for this condition and refuses to attempt the nonsensical.

Reported-by: Su Yue <suy.fnst@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Tested-By: Su Yue <suy.fnst@cn.fujitsu.com>