dmanegine: ioat/dca: Use struct_size() helper

dmanegine: ioat/dca: Use struct_size() helper

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct ioat_dca_priv {
	...
        struct ioat_dca_slot     req_slots[0];
};

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following form:

sizeof(*ioatdca) + (sizeof(struct ioat_dca_slot) * slots)

with:

struct_size(ioatdca, req_slots, slots)

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/20190828184015.GA4273@embeddedor
Signed-off-by: Vinod Koul <vkoul@kernel.org>