selftests: net: add local ECMP rehash test

selftests: net: add local ECMP rehash test

Add ecmp_rehash.sh with nine scenarios verifying that TCP rehash
selects a different local ECMP path for IPv6:

  - SYN retransmission (forward path blocked during setup)
  - SYN/ACK retransmission (reverse path blocked during setup)
  - Midstream RTO (forward path blocked on established connection)
  - Midstream ACK rehash (reverse path blocked on established connection)
  - PLB rehash (ECN-driven congestion on established connection)
  - Hash policy 1 negative test (rehash attempted but path unchanged)
  - No flowlabel leak (client mp_hash does not alter on-wire flowlabel)
  - Dst rebuild consistency (dst invalidation does not change path)
  - Syncookie server path consistency (SYN-ACK and post-cookie ACKs
    use the same ECMP path)

The policy 1 test verifies that fib_multipath_hash_policy=1 computes
a deterministic 5-tuple hash, so txhash re-rolls do not change the
ECMP path while TcpTimeoutRehash still increments.

The flowlabel leak test sets auto_flowlabels=0 on the client and
installs tc filters on client egress that drop TCP packets with
nonzero flowlabel, confirming that the client's fl6->mp_hash does
not leak into the on-wire IPv6 flow label.

The PLB test needs DCTCP, a restricted congestion control.  Rather
than relax the host-global tcp_allowed_congestion_control (no
per-netns equivalent), it pins dctcp on the test routes via the
congctl route attribute, confined to the test namespaces.

The dst rebuild test streams data, invalidates the cached dst by
adding and removing a dummy route (bumping the fib6_node sernum),
and verifies that traffic stays on the same path.  The sernum change
causes ip6_dst_check() to fail on the next transmit, triggering a
fresh route lookup via inet6_csk_route_socket().
ECMP_REBUILD_ROUNDS=10 repeats the check to reduce the probability
of a buggy kernel passing by chance with 2-way ECMP.

The syncookie server path consistency test verifies that the
server's SYN-ACK and subsequent ACKs use the same ECMP path.
With syncookies, the request socket is freed after the SYN-ACK,
so cookie_tcp_reqsk_init() must derive the same txhash (from the
cookie) that was used for the SYN-ACK's route lookup.

The syncookie test forces tcp_syncookies=2; it skips when
CONFIG_SYN_COOKIES is not available.  selftests/net/config selects
it (and CONFIG_TCP_CONG_DCTCP for the PLB test).

Signed-off-by: Neil Spring <ntspring@meta.com>
Link: https://patch.msgid.link/20260615042158.1600746-3-ntspring@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>