selftests: Add an occupancy test for RIF MAC profiles

When all the RIF MAC profiles are in use, test that it is possible to
change the MAC of a netdev (i.e., a RIF) when its MAC profile is not
shared with other RIFs. Test that replacement fails when the MAC profile
is shared.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>