selftests: mptcp: join: validate 8x8 subflows

selftests: mptcp: join: validate 8x8 subflows

The limits have been recently increased, it is required to validate that
having 64 subflows is allowed.

Here, both the client and the server have 8 network interfaces. The
server has 8 endpoints marked as 'signal' to announce all its v4
addresses. The client also has 8 endpoints, but marked as 'subflow' and
'fullmesh' in order to create 8 subflows to each address announced by
the server. This means 63 additional subflows will be created after the
initial one.

If it is not possible to increase the limits to 64, it means an older
kernel version is being used, and the test is skipped.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260508-net-next-mptcp-pm-inc-limits-v1-6-c84e3fdf9b6a@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>