tipc: add media get/dump to new netlink api

Add TIPC_NL_MEDIA_GET command to the new tipc netlink API.

This command supports dumping all information about all defined
media as well as getting all information about a specific media.

The information about a media includes name and link properties.

Netlink logical layout of media get response message:
-> media
    -> name
    -> link properties
        -> tolerance
        -> priority
        -> window

Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@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>