nvmet-rdma: Fix possible NULL deref when handling rdma cm events

When we initiate queue teardown sequence we call rdma_destroy_qp
which clears cm_id->qp, afterwards we call rdma_destroy_id, but
we might see a rdma_cm event in between with a cleared cm_id->qp
so watch out for that and silently ignore the event because this
means that the queue teardown sequence is in progress.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>