selftests/bpf: Test changing packet data from global functions with a kfunc

The verifier should invalidate all packet pointers after a packet data
changing kfunc is called. So, similar to commit 3f23ee55
("selftests/bpf: test for changing packet data from global functions"),
test changing packet data from global functions to make sure packet
pointers are indeed invalidated.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250925170013.1752561-2-ameryhung@gmail.com