Add TIPC_NL_BEARER_GET command to the new tipc netlink API.

This command supports dumping all data about all bearers or getting
all information about a specific bearer.

The information about a bearer includes name, link priorities and
domain.

Netlink logical layout of bearer get message:
-> bearer
    -> name

Netlink logical layout of returned bearer information:
-> bearer
    -> name
    -> link properties
        -> priority
        -> tolerance
        -> window
    -> domain
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>