Input: ff-core - prefer struct_size over open coded arithmetic

Input: ff-core - 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 "ff" variable is a pointer to "struct ff_device" and this
structure ends in a flexible array:

struct ff_device {
	[...]
	struct file *effect_owners[] __counted_by(max_effects);
};

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

The struct_size() helper returns SIZE_MAX on overflow. So, refactor
the comparison to take advantage of this.

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
Link: https://github.com/KSPP/linux/issues/160
Signed-off-by: Erick Archer <erick.archer@outlook.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/AS8PR02MB72371E646714BAE2E51A6A378B152@AS8PR02MB7237.eurprd02.prod.outlook.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>