refscale: Use kcalloc() instead of kzalloc()

Use kcalloc() in main_func() to gain built-in overflow protection, making
memory allocation safer when calculating allocation size compared to
explicit multiplication.

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>