rcutorture: Make failure indication note reader-batch overflow

The loop scanning the pipesummary[] array currently skips the last
element, which means that the diagnostics ignore those rarest of
situations, namely where some readers persist across more than ten
grace periods, but all other readers avoid spanning a full grace period.
This commit therefore adjusts the scan to include the last element of
this array.

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