selftests: mptcp: fix mibit vs mbit mix up

The estimated time was supposing the rate was expressed in mibit
(bit * 1024^2) but it is in mbit (bit * 1000^2).

This makes the threshold higher but in a more realistic way to avoid
false positives reported by CI instances.

Before this patch, the thresholds were at 7561/4005ms and now they are
at 7906/4178ms.

While at it, also fix a typo in the linked comment, spotted by Mat.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/310
Fixes: 1a418cb ("mptcp: simult flow self-tests")
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/simult_flows.sh

@@ -247,9 +247,10 @@ run_test()
 	tc -n $ns2 qdisc add dev ns2eth1 root netem rate ${rate1}mbit $delay1
 	tc -n $ns2 qdisc add dev ns2eth2 root netem rate ${rate2}mbit $delay2
 
-	# time is measured in ms, account for transfer size, affegated link speed
+	# time is measured in ms, account for transfer size, aggregated link speed
 	# and header overhead (10%)
-	local time=$((size * 8 * 1000 * 10 / (( $rate1 + $rate2) * 1024 *1024 * 9) ))
+	#              ms    byte -> bit   10%        mbit      -> kbit -> bit  10%
+	local time=$((1000 * size  *  8  * 10 / ((rate1 + rate2) * 1000 * 1000 * 9) ))
 
 	# mptcp_connect will do some sleeps to allow the mp_join handshake
 	# completion (see mptcp_connect): 200ms on each side, add some slack