torture: Move rcutorture_sched_setaffinity() out of rcutorture

torture: Move rcutorture_sched_setaffinity() out of rcutorture

The rcutorture_sched_setaffinity() function is needed by locktorture,
so move its declaration from rcu.h to torture.h and rename it to the
more generic torture_sched_setaffinity() name.

Please note that use of this function is still restricted to torture
tests, and of those, currently only rcutorture and locktorture.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>