bpf: Move off_reg into sanitize_ptr_alu

bpf: Move off_reg into sanitize_ptr_alu

commit 6f55b2f2a1178856c19bbce2f71449926e731914 upstream.

Small refactor to drag off_reg into sanitize_ptr_alu(), so we later on can
use off_reg for generalizing some of the checks for all pointer types.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
[fllinden@amazon.com: fix minor contextual conflict for 4.14]
Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>