net/rds: don't let the rdma_cm destroy an id RDS still owns on route failure

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

net/rds: don't let the rdma_cm destroy an id RDS still owns on route failure

rds_rdma_cm_event_handler_cmn() hands the return value of
rdma_resolve_route() straight back to the rdma_cm on
RDMA_CM_EVENT_ADDR_RESOLVED. A synchronous failure there - -ENOMEM
from the route work allocation, an SA query that cannot be set up, a
RoCE route with no usable device - makes addr_handler() destroy the
id the event was delivered on. That id is ic->i_cm_id, and nothing
clears the pointer: the connection sits in RDS_CONN_CONNECTING with a
freed id until its shutdown calls rdma_disconnect() and
rdma_destroy_id() on it.

The ROUTE_RESOLVED case already avoids this: rds_ib_cm_initiate_connect()
forces a zero return while ic->i_cm_id == cm_id, as the comment there
explains. Do the same here - drop the connection and return 0 - so
the id stays RDS's to destroy from the shutdown, and the reconnect
gets a fresh one.