rtnetlink: Handle IFLA_MASTER parameter when processing rtnl_newlink

Allow a master interface to be specified as one of the parameters when
creating a new interface via rtnl_newlink.  Previously this would
require invoking interface creation, waiting for it to complete, and
then separately binding that new interface to a master.

In particular, this is used when creating a macvlan child interface for
VRRP in a VRF configuration, allowing the interface creator to specify
directly what master interface should be inherited by the child,
without having to deal with asynchronous complications and potential
race conditions.

Signed-off-by: Theuns Verwoerd <theuns.verwoerd@alliedtelesis.co.nz>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>