selftests/bpf: make 'dubious pointer arithmetic' test useful

mostly revert the previous workaround and make
'dubious pointer arithmetic' test useful again.
Use (ptr - ptr) << const instead of ptr << const to generate large scalar.
The rest stays as before commit 2b36047e.

Fixes: 2b36047e ("selftests/bpf: fix test_align")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>