net: Introduce VRF related flags and helpers

Add a VRF_MASTER flag for interfaces and helper functions for determining
if a device is a VRF_MASTER.

Add link attribute for passing VRF_TABLE id.

Add vrf_ptr to netdevice.

Add various macros for determining if a device is a VRF device, the index
of the master VRF device and table associated with VRF device.

Signed-off-by: Shrijeet Mukherjee <shm@cumulusnetworks.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>