selftests: mptcp: drop msg argument of chk_csum_nr

This patch dropped the msg argument of chk_csum_nr, to unify chk_csum_nr
with other chk_*_nr functions.

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