torture: Notice if an all-zero cpumask is passed inside a critical section

In torture_shuffle_tasks function, the check if an all-zero mask can
be passed to set_cpus_allowed_ptr() is redundant after clearing the
shuffle_idle_cpu bit. If the mask had more than one bit set, after
clearing a bit it has at least one bit set. If the mask had only
one bit set, a check is made at the beginning, where the function
returns, as there is no need to shuffle only one cpu.

Also, this code is executed inside a critical section, delimited by
get_online_cpus(), and put_online_cpus(), preventing CPUs from leaving between
the check of num_online_cpus and the calls to set_cpus_allowed_ptr() function.

Signed-off-by: Iulia Manda <iulia.manda21@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>