selftests: net: Remove executable bits from library scripts

setup_loopback.sh and net_helper.sh are meant to be sourced from other
scripts, not executed directly. Therefore, remove the executable bits from
those files' permissions.

This change is similar to commit 49078c1b80b6 ("selftests: forwarding:
Remove executable bits from lib.sh")

Fixes: 7d1575014a63 ("selftests/net: GRO coalesce test")
Fixes: 3bdd9fd29cb0 ("selftests/net: synchronize udpgro tests' tx and rx connection")
Suggested-by: Paolo Abeni <[e-mail]>
Signed-off-by: Benjamin Poirier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>