selftests: mptcp: add missing oflag=append

selftests: mptcp: add missing oflag=append

In mptcp_connect.sh we are missing something like "oflag=append"
because this will write "${rem}" bytes at the beginning of the file
where there is already some random bytes. It should write that at
the end.

This patch adds this missing 'oflag=append' flag for 'dd' command in
make_file().

Suggested-by: Matthieu Baerts <matttbe@kernel.org>
Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231128-send-net-next-2023107-v4-12-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>