CVE-2026-31413: bpf: Fix unsound scalar forking in maybe_fork_scalars() for BPF_OR

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix unsound scalar forking in maybeforkscalars() for BPFOR

maybeforkscalars() is called for both BPFAND and BPFOR when the source operand is a constant. When dst has signed range [-1, 0], it forks the verifier state: the pushed path gets dst = 0, the current path gets dst = -1.

For BPFAND this is correct: 0 & K == 0. For BPFOR this is wrong: 0 | K == K, not 0.

The pushed path therefore tracks dst as 0 when the runtime value is K, producing an exploitable verifier/runtime divergence that allows out-of-bounds map access.

Fix this by passing env->insnidx (instead of env->insnidx + 1) to pushstack(), so the pushed path re-executes the ALU instruction with dst = 0 and naturally computes the correct result for any opcode.