CVE-2026-74309: vdpa/octeon_ep: fix IRQ-to-ring mapping in interrupt handler

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

vdpa/octeonep: fix IRQ-to-ring mapping in interrupt handler

Look up the IRQ index in octhw->irqs instead of assuming irq - irqs[0]. This supports non-contiguous IRQ numbers and avoids incorrect ring indexing when irqs[0] is not the base.