torture: Seed torture_random_state on CPU

The DEFINE_TORTURE_RANDOM_PERCPU() macro defines per-CPU random-number
generators for torture testing, but the seeds for each CPU's instance
will be identical if they are first used at the same time.  This commit
therefore adds the CPU number to the mix when reseeding.

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