net: mana: validate rx_req_idx to prevent out-of-bounds array access

In the Linux kernel, the following vulnerability has been resolved: net: mana: validate rx_req_idx to prevent out-of-bounds array access In mana_hwc_rx_event_handler(), rx_req_idx is derived from sge->address in DMA-coherent memory. In Confidential VMs (SEV-SNP/TDX), this memory is shared unencrypted and HW can modify WQE contents at any time. No bounds check exists on rx_req_idx, which can lead to an out-of-bounds access into reqs[]. Add bounds check on rx_req_idx in mana_hwc_rx_event_handler() before using it to index the reqs[] array.