selftests: mptcp: simult_flows: adapt limits

selftests: mptcp: simult_flows: adapt limits

Avoid using a fixed limit, no matter the setup. This was causing too
high bufferbloat in some situations, e.g. with a low bandwidth and very
low delay because the default limit was too high for this case.

Instead, use more appropriated limits. Note that unbalanced bandwidth
modes seem to require slightly higher limits to cope with the different
bursts.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260527-net-mptcp-sft-bufferbloat-exit-v1-2-9afc4e742090@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>