selftests: drv-net: rss_ctx: allow more noise on default context

As predicted by David running the test on a machine with a single
interface is a bit unreliable. We try to send 20k packets with
iperf and expect fewer than 10k packets on the default context.
The test isn't very quick, iperf will usually send 100k packets
by the time we stop it. So we're off by 5x on the number of iperf
packets but still expect default context to only get the hardcoded
10k. The intent is to make sure we get noticeably less traffic
on the default context. Use half of the resulting iperf traffic
instead of the hard coded 10k.

Link: https://patch.msgid.link/20240702233728.4183387-1-kuba@kernel.org


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