rcutorture: Pull rcu_torture_updown() loop body into new function

rcutorture: Pull rcu_torture_updown() loop body into new function

This is strictly a code-movement commit, pulling that part of
the rcu_torture_updown() function's loop body that processes
one rcu_torture_one_read_state_updown structure into a new
rcu_torture_updown_one() function.  The checks for the end of the
torture test and the current structure being in use remain in the
rcu_torture_updown() function.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>