bpf: fix liveness propagation to parent in spilled stack slots

Using parent->regs[] when propagating REG_LIVE_READ for spilled regs
doesn't work since parent->regs[] denote the set of normal registers
but not spilled ones. Propagate to the correct regs.

Fixes: dc503a8ad984 ("bpf/verifier: track liveness for pruning")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/verifier.c

@@ -3460,7 +3460,7 @@ static bool do_propagate_liveness(const struct bpf_verifier_state *state,
 		if (parent->spilled_regs[i].live & REG_LIVE_READ)
 			continue;
 		if (state->spilled_regs[i].live == REG_LIVE_READ) {
-			parent->regs[i].live |= REG_LIVE_READ;
+			parent->spilled_regs[i].live |= REG_LIVE_READ;
 			touched = true;
 		}
 	}