Input: i8042 - fix crash at boot time

The driver checks port->exists twice in i8042_interrupt(), first when
trying to assign temporary "serio" variable, and second time when deciding
whether it should call serio_interrupt(). The value of port->exists may
change between the 2 checks, and we may end up calling serio_interrupt()
with a NULL pointer:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000050
IP: [<ffffffff8150feaf>] _spin_lock_irqsave+0x1f/0x40
PGD 0
Oops: 0002 [#1] SMP
last sysfs file:
CPU 0
Modules linked in:

Pid: 1, comm: swapper Not tainted 2.6.32-358.el6.x86_64 #1 QEMU Standard PC (i440FX + PIIX, 1996)
RIP: 0010:[<ffffffff8150feaf>]  [<ffffffff8150feaf>] _spin_lock_irqsave+0x1f/0x40
RSP: 0018:ffff880028203cc0  EFLAGS: 00010082
RAX: 0000000000010000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000282 RSI: 0000000000000098 RDI: 0000000000000050
RBP: ffff880028203cc0 R08: ffff88013e79c000 R09: ffff880028203ee0
R10: 0000000000000298 R11: 0000000000000282 R12: 0000000000000050
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000098
FS:  0000000000000000(0000) GS:ffff880028200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: 0000000000000050 CR3: 0000000001a85000 CR4: 00000000001407f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process swapper (pid: 1, threadinfo ffff88013e79c000, task ffff88013e79b500)
Stack:
ffff880028203d00 ffffffff813de186 ffffffffffffff02 0000000000000000
<d> 0000000000000000 0000000000000000 0000000000000000 0000000000000098
<d> ffff880028203d70 ffffffff813e0162 ffff880028203d20 ffffffff8103b8ac
Call Trace:
<IRQ>
 [<ffffffff813de186>] serio_interrupt+0x36/0xa0
[<ffffffff813e0162>] i8042_interrupt+0x132/0x3a0
[<ffffffff8103b8ac>] ? kvm_clock_read+0x1c/0x20
[<ffffffff8103b8b9>] ? kvm_clock_get_cycles+0x9/0x10
[<ffffffff810e1640>] handle_IRQ_event+0x60/0x170
[<ffffffff8103b154>] ? kvm_guest_apic_eoi_write+0x44/0x50
[<ffffffff810e3d8e>] handle_edge_irq+0xde/0x180
[<ffffffff8100de89>] handle_irq+0x49/0xa0
[<ffffffff81516c8c>] do_IRQ+0x6c/0xf0
[<ffffffff8100b9d3>] ret_from_intr+0x0/0x11
[<ffffffff81076f63>] ? __do_softirq+0x73/0x1e0
[<ffffffff8109b75b>] ? hrtimer_interrupt+0x14b/0x260
[<ffffffff8100c1cc>] ? call_softirq+0x1c/0x30
[<ffffffff8100de05>] ? do_softirq+0x65/0xa0
[<ffffffff81076d95>] ? irq_exit+0x85/0x90
[<ffffffff81516d80>] ? smp_apic_timer_interrupt+0x70/0x9b
[<ffffffff8100bb93>] ? apic_timer_interrupt+0x13/0x20

To avoid the issue let's change the second check to test whether serio is
NULL or not.

Also, let's take i8042_lock in i8042_start() and i8042_stop() instead of
trying to be overly smart and using memory barriers.

Signed-off-by: Chen Hong <chenhong3@huawei.com>
[dtor: take lock in i8042_start()/i8042_stop()]
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>