selftests: drv-net: use defer in the ping test

Make sure the test cleans up after itself. The XDP off statements
at the end of the test may not be reached.

Fixes: 75cc19c8 ("selftests: drv-net: add xdp cases for ping.py")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250312131040.660386-1-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>