Merge branch 'selftests-mptcp-connect-cover-alt-modes'

Matthieu Baerts says:

====================
selftests: mptcp: connect: cover alt modes

mptcp_connect.sh can be executed manually with "-m <MODE>" and "-C" to
make sure everything works as expected when using "mmap" and "sendfile"
modes instead of "poll", and with the MPTCP checksum support.

These modes should be validated, but they are not when the selftests are
executed via the kselftest helpers. It means that most CIs validating
these selftests, like NIPA for the net development trees and LKFT for
the stable ones, are not covering these modes.

To fix that, new test programs have been added, simply calling
mptcp_connect.sh with the right parameters.

The first patch can be backported up to v5.6, and the second one up to
v5.14.

v1: https://lore.kernel.org/20250714-net-mptcp-sft-connect-alt-v1-0-bf1c5abbe575@kernel.org
====================

Link: https://patch.msgid.link/20250715-net-mptcp-sft-connect-alt-v2-0-8230ddd82454@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>