Merge branch 'selftests-net-configure-rp_filter-in-setup_ns'

Hangbin Liu says:

====================
selftests: net: configure rp_filter in setup_ns

Some distributions enable rp_filter globally by default, which can interfere
with various test cases. To address this, many tests explicitly disable
rp_filter within their scripts.

To avoid duplication and ensure consistent behavior across tests, this patch
moves the rp_filter configuration into setup_ns, applied immediately after a
new namespace is created. This change ensures that all namespace-based tests
inherit the appropriate rp_filter settings, simplifying individual test
scripts and improving maintainability.
====================

Link: https://patch.msgid.link/20250508081910.84216-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>