selftests: mptcp: join chk_stale_nr: avoid dup stats

selftests: mptcp: join chk_stale_nr: avoid dup stats

nstat outputs are already printed when calling 'fail_test', no need to
do it again.

While at it, no need to use the dump_stats variable, print the extra
stats directly. And use 'ip -n $ns' instead of 'ip netns exec $ns',
shorter and clearer.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260203-net-next-mptcp-misc-feat-6-20-v1-12-31ec8bfc56d1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>