Merge branch 'mptcp-only-inc-mpjoinackhmacfailure-for-hmac-failures'

Matthieu Baerts says:

====================
mptcp: only inc MPJoinAckHMacFailure for HMAC failures

Recently, during a debugging session using local MPTCP connections, I
noticed MPJoinAckHMacFailure was strangely not zero on the server side.

The first patch fixes this issue -- present since v5.9 -- and the second
one validates it in the selftests.
====================

Link: https://patch.msgid.link/20250407-net-mptcp-hmac-failure-mib-v1-0-3c9ecd0a3a50@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>