RDMA/cxgb4: Fix race in fini path

There exists a race condition where the app disconnects, which
initiates an orderly close (via rdma_fini()), concurrently with an
ingress abort condition, which initiates an abortive close operation.
Since rdma_fini() must be called without IRQs disabled, the fini can
be called after the QP has been transitioned to ERROR.  This is ok,
but we need to protect against qp->ep getting NULLed.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>