selftests: drv-net: Reload pkt pointer after calling filter_udphdr

selftests: drv-net: Reload pkt pointer after calling filter_udphdr

Fix a verification failure. filter_udphdr() calls bpf_xdp_pull_data(),
which will invalidate all pkt pointers. Therefore, all ctx->data loaded
before filter_udphdr() cannot be used. Reload it to prevent verification
errors.

The error may not appear on some compiler versions if they decide to
load ctx->data after filter_udphdr() when it is first used.

Fixes: efec2e55bdef ("selftests: drv-net: Pull data before parsing headers")
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250925161452.1290694-1-ameryhung@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>