SUNRPC: Don't call __UDPX_INC_STATS() from a preemptible context

Calling __UDPX_INC_STATS() from a preemptible context leads to a
warning of the form:

 BUG: using __this_cpu_add() in preemptible [00000000] code: kworker/u5:0/31
 caller is xs_udp_data_receive_workfn+0x194/0x270
 CPU: 1 PID: 31 Comm: kworker/u5:0 Not tainted 4.15.0-rc8-00076-g90ea9f1b #2
 Workqueue: xprtiod xs_udp_data_receive_workfn
 Call Trace:
  dump_stack+0x85/0xc1
  check_preemption_disabled+0xce/0xe0
  xs_udp_data_receive_workfn+0x194/0x270
  process_one_work+0x318/0x620
  worker_thread+0x20a/0x390
  ? process_one_work+0x620/0x620
  kthread+0x120/0x130
  ? __kthread_bind_mask+0x60/0x60
  ret_from_fork+0x24/0x30

Since we're taking a spinlock in those functions anyway, let's fix the
issue by moving the call so that it occurs under the spinlock.
Reported-by: kernel test robot <fengguang.wu@intel.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/xprtsock.c

@@ -1068,18 +1068,18 @@ static void xs_udp_data_read_skb(struct rpc_xprt *xprt,
 
 	/* Suck it into the iovec, verify checksum if not done by hw. */
 	if (csum_partial_copy_to_xdr(&rovr->rq_private_buf, skb)) {
-		__UDPX_INC_STATS(sk, UDP_MIB_INERRORS);
 		spin_lock(&xprt->recv_lock);
+		__UDPX_INC_STATS(sk, UDP_MIB_INERRORS);
 		goto out_unpin;
 	}
 
-	__UDPX_INC_STATS(sk, UDP_MIB_INDATAGRAMS);
 
 	spin_lock_bh(&xprt->transport_lock);
 	xprt_adjust_cwnd(xprt, task, copied);
 	spin_unlock_bh(&xprt->transport_lock);
 	spin_lock(&xprt->recv_lock);
 	xprt_complete_rqst(task, copied);
+	__UDPX_INC_STATS(sk, UDP_MIB_INDATAGRAMS);
 out_unpin:
 	xprt_unpin_rqst(rovr);
  out_unlock: