selftests: fdb_flush: Add test cases for FDB flush with bridge device

Extend the test to check flushing with bridge device, test flush by device
and by VID.

Add test case for flushing with "self" and "master" and attributes that are
supported only in one driver, this is unrecommended configuration, check it
to verify that user gets an error.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>