mtd: maps: sa1100-flash: Prefer struct_size over open coded arithmetic

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

As the "info" variable is a pointer to "struct sa_info" and this
structure ends in a flexible array:

struct sa_info {
	[...]
	struct sa_subdev_info	subdev[];
};

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the calculation "size + size * count" in
the kzalloc() function.

This way, the code is more readable and safer.

This code was detected with the help of Coccinelle, and audited and
modified manually.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/160

 [2]
Signed-off-by: Erick Archer <erick.archer@outlook.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/AS8PR02MB7237AC633B0D1D2EBD3C40E98B392@AS8PR02MB7237.eurprd02.prod.outlook.com