selftests: mptcp: netlink: drop disable=SC2086

Now there are only a few of variables are not using double quotes.
Modifying them, then "shellcheck disable=SC2086" can be dropped.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>