percpu: convert flexible array initializers to use struct_size()

Use the safer macro as sparked by the long discussion in [1].

[1] https://lore.kernel.org/lkml/20200917204514.GA2880159@google.com/

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Dennis Zhou <dennis@kernel.org>