SUNRPC: The RDMA back channel mustn't disappear while requests are outstanding

If there are RDMA back channel requests being processed by the
server threads, then we should hold a reference to the transport
to ensure it doesn't get freed from underneath us.

Reported-by: Neil Brown <[e-mail]>
Fixes: 63cae47005af ("xprtrdma: Handle incoming backward direction RPC calls")
Signed-off-by: Trond Myklebust <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

net/sunrpc/xprtrdma/backchannel.c

···
 	spin_lock(&xprt->bc_pa_lock);
 	list_add_tail(&rqst->rq_bc_pa_list, &xprt->bc_pa_list);
 	spin_unlock(&xprt->bc_pa_lock);
+	xprt_put(xprt);
 }
 
 static struct rpc_rqst *rpcrdma_bc_rqst_get(struct rpcrdma_xprt *r_xprt)
···
 
 	/* Queue rqst for ULP's callback service */
 	bc_serv = xprt->bc_serv;
+	xprt_get(xprt);
 	spin_lock(&bc_serv->sv_cb_lock);
 	list_add(&rqst->rq_bc_list, &bc_serv->sv_cb_list);
 	spin_unlock(&bc_serv->sv_cb_lock);