tipc: refactor multicast ip check

Add a function to check if a tipc UDP media address is a multicast
address or not. This is a purely cosmetic change.

Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>