bpf: Selftests, add printk to test_sk_lookup_kern to encode null ptr check

Adding a printk to test_sk_lookup_kern created the reported failure
where a pointer type is checked twice for NULL. Lets add it to the
progs test test_sk_lookup_kern.c so we test the case from C all the
way into the verifier.

We already have printk's in selftests so seems OK to add another one.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/159009170603.6313.1715279795045285176.stgit@john-Precision-5820-Tower