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

rds_ib_cm_connect_complete() runs from the RDMA-CM event handler with
conn->c_cm_lock held.  When the peer negotiates a protocol version
older than RDS_PROTOCOL_COMPAT_VERSION, the handler calls
rds_conn_destroy(), which is only safe in the rmmod path: it
synchronously tears the connection down and flush_work()es the
shutdown work cp_down_w.

That shutdown work (rds_conn_shutdown()) needs cp_cm_lock, 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, CONNECT_ERROR,
DISCONNECTED) use rds_conn_drop(), which marks the connection
RDS_CONN_ERROR and schedules the shutdown work asynchronously.  Use
it here as well.

Fixes: f147dd9eca ("RDS/IB: Disallow connections less than RDS 3.1")
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Cc: stable@vger.kernel.org
Reviewed-by: Allison Henderson <achender@kernel.org>
Signed-off-by: Aohan Mei <henrymei@tencent.com>
Link: https://patch.msgid.link/20260911073436.3542080-1-ljp1205831794@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/rds/ib_cm.c

@@ -115,7 +115,7 @@ void rds_ib_cm_connect_complete(struct rds_connection *conn, struct rdma_cm_even
 				  &conn->c_laddr, &conn->c_faddr,
 				  RDS_PROTOCOL_MAJOR(conn->c_version),
 				  RDS_PROTOCOL_MINOR(conn->c_version));
-			rds_conn_destroy(conn);
+			rds_conn_drop(conn);
 			return;
 		}
 	}