bpf/verifier: disallow pointer subtraction

Subtraction of pointers was accidentally allowed for unpriv programs
by commit 82abbf8d. Revert that part of commit.

Fixes: 82abbf8d ("bpf: do not allow root to mangle valid pointers")
Reported-by: Jann Horn <jannh@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>