selftests: net: use setup_ns for bareudp testing

selftests: net: use setup_ns for bareudp testing

Switch bareudp testing to use setup_ns, which sets up rp_filter by default.
This allows us to remove the manual rp_filter configuration from the script.

Additionally, since setup_ns handles namespace naming and cleanup, we no
longer need a separate cleanup function. We also move the trap setup earlier
in the script, before the test setup begins.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250508081910.84216-4-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>