selftests: bpf: fix test_kfunc_dynptr_param

selftests: bpf: fix test_kfunc_dynptr_param

As verifier now assumes that all kfuncs only takes trusted pointer
arguments, passing 0 (NULL) to a kfunc that doesn't mark the argument as
__nullable or __opt will be rejected with a failure message of: Possibly
NULL pointer passed to trusted arg<n>

Pass a non-null value to the kfunc to test the expected failure mode.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260102180038.2708325-9-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>