net/ipv4/igmp: Use struct_size() helper

Replace IP_SFLSIZE() with struct_size() helper in order to avoid any
potential type mistakes or integer overflows that, in the worst
scenario, could lead to heap overflows.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>