xprtrdma: Release in-flight MRs on disconnect

Dan Aloni reports that when a server disconnects abruptly, a few
memory regions are left DMA mapped. Over time this leak could pin
enough I/O resources to slow or even deadlock an NFS/RDMA client.

I found that if a transport disconnects before pending Send and
FastReg WRs can be posted, the to-be-registered MRs are stranded on
the req's rl_registered list and never released -- since they
weren't posted, there's no Send completion to DMA unmap them.
Reported-by: Dan Aloni <dan@kernelim.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/verbs.c

@@ -933,6 +933,8 @@ static void rpcrdma_req_reset(struct rpcrdma_req *req)
 
 	rpcrdma_regbuf_dma_unmap(req->rl_sendbuf);
 	rpcrdma_regbuf_dma_unmap(req->rl_recvbuf);
+
+	frwr_reset(req);
 }
 
 /* ASSUMPTION: the rb_allreqs list is stable for the duration,