rcutorture: Reduce TREE01 CPU overcommit

The TREE01.boot nr_cpus kernel boot parameter has been set to 43 for
more than seven years, but it can cause RCU CPU stall warnings on arm64,
most of the time involving the stop-machine subsystem.  This should
not be too surprising, given that this causes 43 vCPUs to spin with
interrupts disabled when there are only eight physical CPUs.

The point of this CPU overcommit is to test the ability of expedited RCU
grace period initialization to handle races with incoming CPUs that have
never previously been online.  But limiting to 17 CPUs instead of 43
allows time for this code to be exercised, and eliminates (or at least
greatly reduces) the incidence of RCU CPU stall warnings on arm64.

So this commit therefore sets nr_cpus=17 in TREE01.boot.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>