Add a test for the IPv4 and IPv6 exception caches, covering the
exceptions that are created in response to ICMP errors quoting a UDP
packet.

The topology consists of a host (h1) that reaches a remote host (h2)
via a router (r1), with a second router (r2) attached to the segment
shared by h1 and r1. UDP packets are injected using a packet socket, so
that an ICMP error quoting them is only matched to a socket when one was
opened separately with the same source port. PMTU errors are provoked by
lowering the MTU of the far end of the path and redirects by pointing
r1's route towards h2 back over the segment it received the packet from.

The following is tested for both address families and for both PMTU and
redirect exceptions:

* An error that is not matched to a socket creates an exception that
  carries the new MTU or gateway.
* An error that is matched to a socket creates the same exception.

The PMTU tests further verify that a lower PMTU replaces the one stored
in the exception whereas a higher one does not, and that a socket which
disabled PMTU discovery using IP{,V6}_PMTUDISC_OMIT gets the same
exception as the other cases.

Without "ipv4: udp: Create exceptions before socket matching" and "ipv6:
udp: Create exceptions before socket matching", the tests that do not
open a socket fail:

 # ./exception_cache.sh
 TEST: IPv4: PMTU: exception without a matching socket               [FAIL]
         No socket: exception does not carry an MTU of 1400
 TEST: IPv6: PMTU: exception without a matching socket               [FAIL]
         No socket: exception does not carry an MTU of 1400
 TEST: IPv4: PMTU: exception with a matching socket                  [ OK ]
 TEST: IPv6: PMTU: exception with a matching socket                  [ OK ]
 TEST: IPv4: PMTU: exception with a socket ignoring it               [FAIL]
         PMTU discovery disabled: exception does not carry an MTU of 1400
 TEST: IPv6: PMTU: exception with a socket ignoring it               [FAIL]
         PMTU discovery disabled: exception does not carry an MTU of 1400
 TEST: IPv4: Redirect: exception without a matching socket           [FAIL]
         No socket: exception does not carry the new gateway
 TEST: IPv6: Redirect: exception without a matching socket           [FAIL]
         No socket: exception does not carry the new gateway
 TEST: IPv4: Redirect: exception with a matching socket              [ OK ]
 TEST: IPv6: Redirect: exception with a matching socket              [ OK ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260828192344.2596928-5-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>