Merge branch 'mptcp-selftests-more-debug-in-case-of-errors'

Matthieu Baerts says:

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mptcp: selftests: more debug in case of errors

Here are just a bunch of small improvements for the MPTCP selftests:

Patch 1: Unify errors messages in simult_flows: print MIB and 'ss -Me'.

Patch 2: Unify errors messages in sockopt: print MIB.

Patch 3: Move common code to print debug info to mptcp_lib.sh.

Patch 4: Use 'ss' with '-m' in case of errors.

Patch 5: Remove an unused variable.

Patch 6: Print only the size instead of size + filename again.
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Link: https://patch.msgid.link/20250114-net-next-mptcp-st-more-debug-err-v1-0-2ffb16a6cf35@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>