rcuscale: using kcalloc() to relpace kmalloc()

It's safer to using kcalloc() because it can prevent overflow
problem.

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