tipc: split UDP nl address parsing

Split the UDP netlink parse function so that it only parses one
netlink attribute at the time. This makes the parse function more
generic and allow future UDP API functions to use it for parsing.

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