mtd: cfi: use struct_size() helper for cfiq allocation

Documentation/process/deprecated.rst recommends against performing
dynamic size calculations in the arguments of memory allocator
function due to the risk of overflow. Such calculations can
wrap around and result in a smaller allocation than what the caller
was expecting.

Replace the size calculation in cfiq allocation with struct_size()
helper to make the code clearer and handle the overflows correctly.

Signed-off-by: Gopi Krishna Menon <[e-mail]>
Reviewed-by: Vignesh Raghavendra <[e-mail]>
link: https://lore.kernel.org/linux-kernel-mentees/[e-mail]/T/#u
Signed-off-by: Miquel Raynal <[e-mail]>

drivers/mtd/chips/cfi_probe.c

···
 	if (!num_erase_regions)
 		return 0;
 
-	cfi->cfiq = kmalloc(sizeof(struct cfi_ident) + num_erase_regions * 4, GFP_KERNEL);
+	cfi->cfiq = kmalloc(struct_size(cfi->cfiq, EraseRegionInfo, num_erase_regions), GFP_KERNEL);
 	if (!cfi->cfiq)
 		return 0;