selftests/net: give more time to udpgro bg processes to complete startup

[ Upstream commit cdb525ca ]

In some conditions, background processes in udpgro don't have enough
time to set up the sockets. When foreground processes start, this
results in the test failing with "./udpgso_bench_tx: sendmsg: Connection
refused". For instance, this happens from time to time on a Qualcomm
SA8540P SoC running CentOS Stream 9.

To fix this, increase the time given to background processes to
complete the startup before foreground processes start.

Signed-off-by: Adrien Thierry <athierry@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 9d851dd4 ("selftests: net: Remove executable bits from library scripts")
Signed-off-by: Sasha Levin <sashal@kernel.org>