spi: 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 spi_replaced_transfers {
	...
        struct spi_transfer inserted_transfers[];
};

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:

insert * sizeof(struct spi_transfer) + sizeof(struct spi_replaced_transfers)

with:

struct_size(rxfer, inserted_transfers, insert)

This code was detected with the help of Coccinelle.
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -2750,8 +2750,7 @@ struct spi_replaced_transfers *spi_replace_transfers(
 
 	/* allocate the structure using spi_res */
 	rxfer = spi_res_alloc(msg->spi, __spi_replace_transfers_release,
-			      insert * sizeof(struct spi_transfer)
-			      + sizeof(struct spi_replaced_transfers)
+			      struct_size(rxfer, inserted_transfers, insert)
 			      + extradatasize,
 			      gfp);
 	if (!rxfer)