Merge branch 'selftests-mptcp-increase-code-coverage'

Matthieu Baerts says:

====================
selftests: mptcp: increase code coverage

Here are various patches slightly improving MPTCP code coverage:

- Patch 1: avoid a harmless 'grep: write error' warning.

- Patch 2: use getaddrinfo() with IPPROTO_MPTCP in more places.

- Patch 3-6: prepare and add support to get info for a specific subflow
  when giving the 5-tuple.

- Patch 7: validate the previous patch and cover "subflow_get_info_size"
  in the kernel code (mptcp_diag.c).
====================

Link: https://patch.msgid.link/20250502-net-next-mptcp-sft-inc-cover-v1-0-68eec95898fb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>