selftests: forwarding: Add test for custom multipath hash

Test that when the hash policy is set to custom, traffic is distributed
only according to the outer 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 with non-encapsulated IPv4 and IPv6
packets. Subsequent patches will add tests for IPv4/IPv6 overlays on top
of IPv4/IPv6 underlay networks.

Example output:

 # ./custom_multipath_hash.sh
 TEST: ping                                                          [ OK ]
 TEST: ping6                                                         [ OK ]
 INFO: Running IPv4 custom multipath hash tests
 TEST: Multipath hash field: Source IP (balanced)                    [ OK ]
 INFO: Packets sent on path1 / path2: 6353 / 6254
 TEST: Multipath hash field: Source IP (unbalanced)                  [ OK ]
 INFO: Packets sent on path1 / path2: 0 / 12600
 TEST: Multipath hash field: Destination IP (balanced)               [ OK ]
 INFO: Packets sent on path1 / path2: 6102 / 6502
 TEST: Multipath hash field: Destination IP (unbalanced)             [ OK ]
 INFO: Packets sent on path1 / path2: 1 / 12601
 TEST: Multipath hash field: Source port (balanced)                  [ OK ]
 INFO: Packets sent on path1 / path2: 16428 / 16345
 TEST: Multipath hash field: Source port (unbalanced)                [ OK ]
 INFO: Packets sent on path1 / path2: 32770 / 2
 TEST: Multipath hash field: Destination port (balanced)             [ OK ]
 INFO: Packets sent on path1 / path2: 16428 / 16345
 TEST: Multipath hash field: Destination port (unbalanced)           [ OK ]
 INFO: Packets sent on path1 / path2: 32770 / 2
 INFO: Running IPv6 custom multipath hash tests
 TEST: Multipath hash field: Source IP (balanced)                    [ OK ]
 INFO: Packets sent on path1 / path2: 6704 / 5903
 TEST: Multipath hash field: Source IP (unbalanced)                  [ OK ]
 INFO: Packets sent on path1 / path2: 12600 / 0
 TEST: Multipath hash field: Destination IP (balanced)               [ OK ]
 INFO: Packets sent on path1 / path2: 5551 / 7052
 TEST: Multipath hash field: Destination IP (unbalanced)             [ OK ]
 INFO: Packets sent on path1 / path2: 12603 / 0
 TEST: Multipath hash field: Flowlabel (balanced)                    [ OK ]
 INFO: Packets sent on path1 / path2: 8378 / 8080
 TEST: Multipath hash field: Flowlabel (unbalanced)                  [ OK ]
 INFO: Packets sent on path1 / path2: 2 / 12603
 TEST: Multipath hash field: Source port (balanced)                  [ OK ]
 INFO: Packets sent on path1 / path2: 16385 / 16388
 TEST: Multipath hash field: Source port (unbalanced)                [ OK ]
 INFO: Packets sent on path1 / path2: 0 / 32774
 TEST: Multipath hash field: Destination port (balanced)             [ OK ]
 INFO: Packets sent on path1 / path2: 16386 / 16390
 TEST: Multipath hash field: Destination port (unbalanced)           [ OK ]
 INFO: Packets sent on path1 / path2: 32771 / 2

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