RDMA/ucma: Discard events for IDs not yet claimed by user space

Problem reported by Avneesh Pant <avneesh.pant@oracle.com>:

    It looks like we are triggering a bug in RDMA CM/UCM interaction.
    The bug specifically hits when we have an incoming connection
    request and the connecting process dies BEFORE the passive end of
    the connection can process the request i.e. it does not call
    rdma_get_cm_event() to retrieve the initial connection event.  We
    were able to triage this further and have some additional
    information now.

    In the example below when P1 dies after issuing a connect request
    as the CM id is being destroyed all outstanding connects (to P2)
    are sent a reject message. We see this reject message being
    received on the passive end and the appropriate CM ID created for
    the initial connection message being retrieved in cm_match_req().
    The problem is in the ucma_event_handler() code when this reject
    message is delivered to it and the initial connect message itself
    HAS NOT been delivered to the client. In fact the client has not
    even called rdma_cm_get_event() at this stage so we haven't
    allocated a new ctx in ucma_get_event() and updated the new
    connection CM_ID to point to the new UCMA context.

    This results in the reject message not being dropped in
    ucma_event_handler() for the new connection request as the
    (if (!ctx->uid)) block is skipped since the ctx it refers to is
    the listen CM id context which does have a valid UID associated
    with it (I believe the new CMID for the connection initially
    uses the listen CMID -> context when it is created in
    cma_new_conn_id). Thus the assumption that new events for a
    connection can get dropped in ucma_event_handler() is incorrect
    IF the initial connect request has not been retrieved in the
    first case. We end up getting a CM Reject event on the listen CM
    ID and our upper layer code asserts (in fact this event does not
    even have the listen_id set as that only gets set up librdmacm
    for connect requests).

The solution is to verify that the cm_id being reported in the event
is the same as the cm_id referenced by the ucma context.  A mismatch
indicates that the ucma context corresponds to the listen.  This fix
was validated by using a modified version of librdmacm that was able
to verify the problem and see that the reject message was indeed
dropped after this patch was applied.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>