RX return-ring buffer index validated only by KKASSERT (compiled out without INVARIANTS)

Summary

bnx_rxeof at if_bnx.c:2812-2842: rxidx=cur_rx->bge_idx (NIC-controlled uint16) used to index bnx_rx_std_buf[BGE_STD_RX_RING_CNT=512]. Sole guard is KKASSERT(rxidx<512) which compiles to nothing without INVARIANTS. On INVARIANTS-less kernels OOB read/write of bnx_rx_std_ring trailing fields (DMA tag, ring map). Not directly network-exploitable (NIC firmware controls bge_idx not frame bytes); requires NIC bug or malicious PCIe device. On stock kernel (INVARIANTS on): panic. Fix: replace KKASSERT with hard runtime check.