Convert usage of IN_MULTICAST to ipv4_is_multicast

IN_MULTICAST's primary intent is as a uapi macro.

Elsewhere in the kernel we use ipv4_is_multicast consistently.

This patch unifies linux's multicast checks to use that function
rather than this macro.

Signed-off-by: Dave Taht <dave.taht@gmail.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>