selftests: mptcp: join: fix "invalid address, ADD_ADDR timeout"

The "Fixes" commit mentioned below adds new MIBs counters to track some
particular cases that have been fixed by its parent commit 150d1e06
("mptcp: fix race in incoming ADD_ADDR option processing").

Unfortunately, one of the new MIB counter (AddAddrDrop) shares the same
prefix as an older one (AddAddr). This breaks one selftest because it
was doing a grep on "AddAddr" and it now gets 2 counters instead of 1.

This issue has been fixed upstream in a commit that was part of the same
set but not backported to v5.15, see commit 6ef84b15 ("selftests:
mptcp: more robust signal race test"). It has not been backported
because it was fixing multiple things, some where for >v5.15.

This patch then simply extracts the only bit needed for v5.15. Now the
test passes when validating the last stable v5.15 kernel.

Fixes: f25ae162 ("mptcp: add mibs counter for ignored incoming options")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>