Add a test case for IPv6 source address delete.

As David suggested, add tests:
- Single device using src address
- Two devices with the same source address
- VRF with single device using src address
- VRF with two devices using src address

As Ido points out, in IPv6, the preferred source address is looked up in
the same VRF as the first nexthop device. This will give us similar results
to IPv4 if the route is installed in the same VRF as the nexthop device, but
not when the nexthop device is enslaved to a different VRF. So add tests:
- src address and nexthop dev in same VR
- src address and nexthop device in different VRF

The link local address delete logic is different from the global address.
It should only affect the associate device it bonds to. So add tests cases
for link local address testing.

Here is the test result:

IPv6 delete address route tests
    Single device using src address
    TEST: Prefsrc removed when src address removed on other device      [ OK ]
    Two devices with the same source address
    TEST: Prefsrc not removed when src address exist on other device    [ OK ]
    TEST: Prefsrc removed when src address removed on all devices       [ OK ]
    VRF with single device using src address
    TEST: Prefsrc removed when src address removed on other device      [ OK ]
    VRF with two devices using src address
    TEST: Prefsrc not removed when src address exist on other device    [ OK ]
    TEST: Prefsrc removed when src address removed on all devices       [ OK ]
    src address and nexthop dev in same VRF
    TEST: Prefsrc removed from VRF when source address deleted          [ OK ]
    TEST: Prefsrc in default VRF not removed                            [ OK ]
    TEST: Prefsrc not removed from VRF when source address exist        [ OK ]
    TEST: Prefsrc in default VRF removed                                [ OK ]
    src address and nexthop device in different VRF
    TEST: Prefsrc not removed from VRF when nexthop dev in diff VRF     [ OK ]
    TEST: Prefsrc not removed in default VRF                            [ OK ]
    TEST: Prefsrc removed from VRF when nexthop dev in diff VRF         [ OK ]
    TEST: Prefsrc removed in default VRF                                [ OK ]
    Table ID 0
    TEST: Prefsrc removed from default VRF when source address deleted  [ OK ]
    Link local source route
    TEST: Prefsrc not removed when delete ll addr from other dev        [ OK ]
    TEST: Prefsrc removed when delete ll addr                           [ OK ]
    TEST: Prefsrc not removed when delete ll addr from other dev        [ OK ]
    TEST: Prefsrc removed even ll addr still exist on other dev         [ OK ]

Tests passed:  19
Tests failed:   0
Suggested-by: Ido Schimmel <idosch@idosch.org>
Suggested-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>