SUNRPC: Always manipulate rpc_rqst::rq_bc_pa_list under xprt->bc_pa_lock

Other code that accesses rq_bc_pa_list holds xprt->bc_pa_lock.
xprt_complete_bc_request() should do the same.

Fixes: 2ea24497 ("SUNRPC: RPC callbacks may be split . . .")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/backchannel_rqst.c

@@ -309,12 +309,15 @@ void xprt_complete_bc_request(struct rpc_rqst *req, uint32_t copied)
 	struct rpc_xprt *xprt = req->rq_xprt;
 	struct svc_serv *bc_serv = xprt->bc_serv;
 
+	spin_lock(&xprt->bc_pa_lock);
+	list_del(&req->rq_bc_pa_list);
+	spin_unlock(&xprt->bc_pa_lock);
+
 	req->rq_private_buf.len = copied;
 	set_bit(RPC_BC_PA_IN_USE, &req->rq_bc_pa_state);
 
 	dprintk("RPC:       add callback request to list\n");
 	spin_lock(&bc_serv->sv_cb_lock);
-	list_del(&req->rq_bc_pa_list);
 	list_add(&req->rq_bc_list, &bc_serv->sv_cb_list);
 	wake_up(&bc_serv->sv_cb_waitq);
 	spin_unlock(&bc_serv->sv_cb_lock);