selftests: mptcp: join: validate MPJ SYN TX MIB counters

A few new MPJoinSynTx MIB counters have been added in a previous commit.
They are being validated here in mptcp_join.sh selftest, each time the
number of received MPJ are checked.

Most of the time, the number of sent SYN+MPJ is the same as the received
ones. But sometimes, there are more, because there are dropped, or there
are errors.

While at it, the "no MPC reuse with single endpoint" subtest has been
modified to force a bind() error.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240902-net-next-mptcp-mib-mpjtx-misc-v1-6-d3e0f3773b90@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>