selftests/bpf: Test changing packet data from kfunc

bpf_xdp_pull_data() is the first kfunc that changes packet data. Make
sure the verifier clear all packet pointers after calling packet data
changing kfunc.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250926164142.1850176-1-ameryhung@gmail.com