selftests: forwarding: Add test for custom multipath hash with IPv6 GRE

Test that when the hash policy is set to custom, traffic is distributed
only according to the inner fields set in the fib_multipath_hash_fields
sysctl.

Each time set a different field and make sure traffic is only
distributed when the field is changed in the packet stream.

The test only verifies the behavior of IPv4/IPv6 overlays on top of an
IPv6 underlay network. The previous patch verified the same with an IPv4
underlay network.

Example output:

 # ./ip6gre_custom_multipath_hash.sh
 TEST: ping                                                          [ OK ]
 TEST: ping6                                                         [ OK ]
 INFO: Running IPv4 overlay custom multipath hash tests
 TEST: Multipath hash field: Inner source IP (balanced)              [ OK ]
 INFO: Packets sent on path1 / path2: 6602 / 6002
 TEST: Multipath hash field: Inner source IP (unbalanced)            [ OK ]
 INFO: Packets sent on path1 / path2: 1 / 12601
 TEST: Multipath hash field: Inner destination IP (balanced)         [ OK ]
 INFO: Packets sent on path1 / path2: 6802 / 5801
 TEST: Multipath hash field: Inner destination IP (unbalanced)       [ OK ]
 INFO: Packets sent on path1 / path2: 12602 / 3
 TEST: Multipath hash field: Inner source port (balanced)            [ OK ]
 INFO: Packets sent on path1 / path2: 16431 / 16344
 TEST: Multipath hash field: Inner source port (unbalanced)          [ OK ]
 INFO: Packets sent on path1 / path2: 0 / 32773
 TEST: Multipath hash field: Inner destination port (balanced)       [ OK ]
 INFO: Packets sent on path1 / path2: 16431 / 16344
 TEST: Multipath hash field: Inner destination port (unbalanced)     [ OK ]
 INFO: Packets sent on path1 / path2: 2 / 32772
 INFO: Running IPv6 overlay custom multipath hash tests
 TEST: Multipath hash field: Inner source IP (balanced)              [ OK ]
 INFO: Packets sent on path1 / path2: 6704 / 5902
 TEST: Multipath hash field: Inner source IP (unbalanced)            [ OK ]
 INFO: Packets sent on path1 / path2: 1 / 12600
 TEST: Multipath hash field: Inner destination IP (balanced)         [ OK ]
 INFO: Packets sent on path1 / path2: 5751 / 6852
 TEST: Multipath hash field: Inner destination IP (unbalanced)       [ OK ]
 INFO: Packets sent on path1 / path2: 12602 / 0
 TEST: Multipath hash field: Inner flowlabel (balanced)              [ OK ]
 INFO: Packets sent on path1 / path2: 8272 / 8181
 TEST: Multipath hash field: Inner flowlabel (unbalanced)            [ OK ]
 INFO: Packets sent on path1 / path2: 3 / 12602
 TEST: Multipath hash field: Inner source port (balanced)            [ OK ]
 INFO: Packets sent on path1 / path2: 16424 / 16351
 TEST: Multipath hash field: Inner source port (unbalanced)          [ OK ]
 INFO: Packets sent on path1 / path2: 3 / 32774
 TEST: Multipath hash field: Inner destination port (balanced)       [ OK ]
 INFO: Packets sent on path1 / path2: 16425 / 16350
 TEST: Multipath hash field: Inner destination port (unbalanced)     [ OK ]
 INFO: Packets sent on path1 / path2: 2 / 32773
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>