Add extack arg to the nla_parse_nested calls in rtnl_newlink, and
add messages for unknown device type and link network namespace id.
In particular, it improves the failure message when the wrong link
type is used. From
    $ ip li add bond1 type bonding
    RTNETLINK answers: Operation not supported
to
    $ ip li add bond1 type bonding
    Error: Unknown device type.

(The module name is bonding but the link type is bond.)
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>