selftests: fib_test: Add a test case for IPv4 broadcast neighbours

Test that resolved neighbours for IPv4 broadcast addresses are
unaffected by the configuration of matching broadcast routes, whereas
unresolved neighbours are invalidated.

Without previous patch:

 # ./fib_tests.sh -t ipv4_bcast_neigh

 IPv4 broadcast neighbour tests
     TEST: Resolved neighbour for broadcast address                      [ OK ]
     TEST: Resolved neighbour for network broadcast address              [ OK ]
     TEST: Unresolved neighbour for broadcast address                    [FAIL]
     TEST: Unresolved neighbour for network broadcast address            [FAIL]

 Tests passed:   2
 Tests failed:   2

With previous patch:

 # ./fib_tests.sh -t ipv4_bcast_neigh

 IPv4 broadcast neighbour tests
     TEST: Resolved neighbour for broadcast address                      [ OK ]
     TEST: Resolved neighbour for network broadcast address              [ OK ]
     TEST: Unresolved neighbour for broadcast address                    [ OK ]
     TEST: Unresolved neighbour for network broadcast address            [ OK ]

 Tests passed:   4
 Tests failed:   0

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