bpf/verifier: per-register parent pointers

By giving each register its own liveness chain, we elide the skip_callee()
 logic.  Instead, each register's parent is the state it inherits from;
 both check_func_call() and prepare_func_exit() automatically connect
 reg states to the correct chain since when they copy the reg state across
 (r1-r5 into the callee as args, and r0 out as the return value) they also
 copy the parent pointer.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>