ebpf: verifier: check that call reg with ARG_ANYTHING is initialized

I noticed that a helper function with argument type ARG_ANYTHING does
not need to have an initialized value (register).

This can worst case lead to unintented stack memory leakage in future
helper functions if they are not carefully designed, or unintended
application behaviour in case the application developer was not careful
enough to match a correct helper function signature in the API.

The underlying issue is that ARG_ANYTHING should actually be split
into two different semantics:

  1) ARG_DONTCARE for function arguments that the helper function
     does not care about (in other words: the default for unused
     function arguments), and

  2) ARG_ANYTHING that is an argument actually being used by a
     helper function and *guaranteed* to be an initialized register.

The current risk is low: ARG_ANYTHING is only used for the 'flags'
argument (r4) in bpf_map_update_elem() that internally does strict
checking.

Fixes: 17a52670 ("bpf: verifier (add verifier core)")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>