CVE-2026-98257: rds: ib: use rds_conn_drop() on protocol version mismatch

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

rds: ib: use rdsconndrop() on protocol version mismatch

rdsibcmconnectcomplete() runs from the RDMA-CM event handler with conn->ccmlock held. When the peer negotiates a protocol version older than RDSPROTOCOLCOMPATVERSION, the handler calls rdsconndestroy(), which is only safe in the rmmod path: it synchronously tears the connection down and flushwork()es the shutdown work cpdownw.

That shutdown work (rdsconnshutdown()) needs cpcmlock, which is the very lock the event handler still holds, so the flush never completes: the two workers wait on each other and the RDS connection workqueues stall for good.

All other RDMA-CM failure paths (REJECTED, CONNECTERROR, DISCONNECTED) use rdsconndrop(), which marks the connection RDSCONNERROR and schedules the shutdown work asynchronously. Use it here as well.