selftests: bpf: Update kfunc_param_nullable test for new error message

With trusted args now being the default, the NULL pointer check runs
before type-specific validation. Update test3 to expect the new error
message "Possibly NULL pointer passed to trusted arg0" instead of the
old dynptr-specific error message.

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/20260102180038.2708325-7-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/test_kfunc_param_nullable.c

@@ -29,7 +29,7 @@ int kfunc_dynptr_nullable_test2(struct __sk_buff *skb)
 }
 
 SEC("tc")
-__failure __msg("expected pointer to stack or const struct bpf_dynptr")
+__failure __msg("Possibly NULL pointer passed to trusted arg0")
 int kfunc_dynptr_nullable_test3(struct __sk_buff *skb)
 {
 	struct bpf_dynptr data;