xen/events: fix RCU warning, or Call idle notifier after irq_enter()

exit_idle() should be called after irq_enter(), otherwise it throws:

[ INFO: suspicious RCU usage. ]
3.6.5 #1 Not tainted
-------------------------------
include/linux/rcupdate.h:725 rcu_read_lock() used illegally while idle!

other info that might help us debug this:

RCU used illegally from idle CPU!
rcu_scheduler_active = 1, debug_locks = 1
RCU used illegally from extended quiescent state!
1 lock held by swapper/0/0:
 #0:  (rcu_read_lock){......}, at: [<ffffffff810e9fe0>] __atomic_notifier_call_chain+0x0/0x140

stack backtrace:
Pid: 0, comm: swapper/0 Not tainted 3.6.5 #1
Call Trace:
 <IRQ>  [<ffffffff811259a2>] lockdep_rcu_suspicious+0xe2/0x130
 [<ffffffff810ea10c>] __atomic_notifier_call_chain+0x12c/0x140
 [<ffffffff810e9fe0>] ? atomic_notifier_chain_unregister+0x90/0x90
 [<ffffffff811216cd>] ? trace_hardirqs_off+0xd/0x10
 [<ffffffff810ea136>] atomic_notifier_call_chain+0x16/0x20
 [<ffffffff810777c3>] exit_idle+0x43/0x50
 [<ffffffff81568865>] xen_evtchn_do_upcall+0x25/0x50
 [<ffffffff81aa690e>] xen_do_hypervisor_callback+0x1e/0x30
 <EOI>  [<ffffffff810013aa>] ? hypercall_page+0x3aa/0x1000
 [<ffffffff810013aa>] ? hypercall_page+0x3aa/0x1000
 [<ffffffff81061540>] ? xen_safe_halt+0x10/0x20
 [<ffffffff81075cfa>] ? default_idle+0xba/0x570
 [<ffffffff810778af>] ? cpu_idle+0xdf/0x140
 [<ffffffff81a4d881>] ? rest_init+0x135/0x144
 [<ffffffff81a4d74c>] ? csum_partial_copy_generic+0x16c/0x16c
 [<ffffffff82520c45>] ? start_kernel+0x3db/0x3e8
 [<ffffffff8252066a>] ? repair_env_string+0x5a/0x5a
 [<ffffffff82520356>] ? x86_64_start_reservations+0x131/0x135
 [<ffffffff82524aca>] ? xen_start_kernel+0x465/0x46

Git commit 98ad1cc1
Author: Frederic Weisbecker <fweisbec@gmail.com>
Date:   Fri Oct 7 18:22:09 2011 +0200

    x86: Call idle notifier after irq_enter()

did this, but it missed the Xen code.
Signed-off-by: Mojiong Qiu <mjqiu@tencent.com>
Cc: stable@vger.kernel.org # from 3.3 and newer.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/events.c

@@ -1395,10 +1395,10 @@ void xen_evtchn_do_upcall(struct pt_regs *regs)
 {
 	struct pt_regs *old_regs = set_irq_regs(regs);
 
+	irq_enter();
 #ifdef CONFIG_X86
 	exit_idle();
 #endif
-	irq_enter();
 
 	__xen_evtchn_do_upcall();