selftests: mptcp: add local variables rndh

This patch adds local variables rndh in do_transfer() functions both in
mptcp_connect.sh and simult_flows.sh, setting it with ${ns1:4}, not the
global variable rndh. The global one is hidden in the next commit.

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>
Link: https://lore.kernel.org/r/20240306-upstream-net-next-20240304-selftests-mptcp-shared-code-shellcheck-v2-3-bc79e6e5e6a0@kernel.org


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