ipv4: nexthop: Reduce allocation size of 'struct nh_group'

The struct looks as follows:

struct nh_group {
	struct nh_group		*spare; /* spare group for removals */
	u16			num_nh;
	bool			mpath;
	bool			fdb_nh;
	bool			has_v4;
	struct nh_grp_entry	nh_entries[];
};

But its offset within 'struct nexthop' is also taken into account to
determine the allocation size.

Instead, use struct_size() to allocate only the required number of
bytes.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/nexthop.c

@@ -133,12 +133,9 @@ static struct nexthop *nexthop_alloc(void)
 
 static struct nh_group *nexthop_grp_alloc(u16 num_nh)
 {
-	size_t sz = offsetof(struct nexthop, nh_grp)
-		    + sizeof(struct nh_group)
-		    + sizeof(struct nh_grp_entry) * num_nh;
 	struct nh_group *nhg;
 
-	nhg = kzalloc(sz, GFP_KERNEL);
+	nhg = kzalloc(struct_size(nhg, nh_entries, num_nh), GFP_KERNEL);
 	if (nhg)
 		nhg->num_nh = num_nh;