selftests: net: disable rp_filter after namespace initialization

selftests: net: disable rp_filter after namespace initialization

Some distributions enable rp_filter globally by default. To ensure consistent
behavior across environments, we explicitly disable it in several test cases.

This patch moves the rp_filter disabling logic to immediately after the
network namespace is initialized. With this change, individual test cases
with creating namespace via setup_ns no longer need to disable rp_filter
again.

This helps avoid redundancy and ensures test consistency.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250508081910.84216-2-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>