selftests: drv-net: xdp: Fix register spill error with clang 20

selftests: drv-net: xdp: Fix register spill error with clang 20

On clang 20.1.8 the XDP program fails to load with a register spill error.
Since hdr_len is a __u32, the compiler decided it only needed the lower
32-bits of ctx->data, which later triggers the register spill verifier
error.

Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Dimitri Daskalakis <dimitri.daskalakis1@gmail.com>
Link: https://patch.msgid.link/20251113043102.4062150-1-dimitri.daskalakis1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>