selftests: mptcp: clearly declare global ns vars

It is clearer to declare these global variables at the beginning of the
file as it is done in other MPTCP selftests rather than in functions in
the middle of the script.

So for uniformity reason, we can do the same here in mptcp_sockopt.sh.

Suggested-by: Geliang Tang <geliang.tang@suse.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>