libceph: use struct_size() for kmalloc() in crush_decode()

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 foo {
    int stuff;
    struct boo entry[];
};

instance = kmalloc(sizeof(struct foo) + count * sizeof(struct boo), GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

instance = kmalloc(struct_size(instance, entry, count), GFP_KERNEL);

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/osdmap.c

@@ -495,9 +495,8 @@ static struct crush_map *crush_decode(void *pbyval, void *end)
 			  / sizeof(struct crush_rule_step))
 			goto bad;
 #endif
-		r = c->rules[i] = kmalloc(sizeof(*r) +
-					  yes*sizeof(struct crush_rule_step),
-					  GFP_NOFS);
+		r = kmalloc(struct_size(r, steps, yes), GFP_NOFS);
+		c->rules[i] = r;
 		if (r == NULL)
 			goto badmem;
 		dout(" rule %d is at %p\n", i, r);