pci_get_class(class, from) drops the refcount for 'from', so the
extra pci_dev_put we do on it will result in a use after free bug
starting with the WARN below.

Regression introduced in

commit 6a9c4b35
Author: Rui Guo <firemeteor@users.sourceforge.net>
Date:   Wed Jun 19 21:10:23 2013 +0800

    drm/i915: Fix PCH detect with multiple ISA bridges in VM

[  164.338460] WARNING: CPU: 1 PID: 2094 at include/linux/kref.h:47 klist_next+0xae/0x110()
[  164.347731] CPU: 1 PID: 2094 Comm: modprobe Tainted: G           O 3.13.0-imre+ #354
[  164.356468] Hardware name: Intel Corp. VALLEYVIEW B0 PLATFORM/NOTEBOOK, BIOS BYTICRB1.X64.0062.R70.1310112051 10/11/2013
[  164.368796] Call Trace:
[  164.371609]  [<ffffffff816a32a6>] dump_stack+0x4e/0x7a
[  164.377447]  [<ffffffff8104f75d>] warn_slowpath_common+0x7d/0xa0
[  164.384238]  [<ffffffff8104f83a>] warn_slowpath_null+0x1a/0x20
[  164.390851]  [<ffffffff8169aeae>] klist_next+0xae/0x110
[  164.396777]  [<ffffffff8130a110>] ? pci_do_find_bus+0x70/0x70
[  164.403286]  [<ffffffff813cb4a9>] bus_find_device+0x89/0xc0
[  164.409719]  [<ffffffff8130a373>] pci_get_dev_by_id+0x63/0xa0
[  164.416238]  [<ffffffff8130a4e4>] pci_get_class+0x44/0x50
[  164.422433]  [<ffffffffa034821f>] intel_dsm_detect+0x16f/0x1f0 [i915]
[  164.429801]  [<ffffffffa03482ae>] intel_register_dsm_handler+0xe/0x10 [i915]
[  164.437831]  [<ffffffffa02d30fe>] i915_driver_load+0xafe/0xf30 [i915]
[  164.445126]  [<ffffffff8158a150>] ? intel_alloc_coherent+0x110/0x110
[  164.452340]  [<ffffffffa0148c07>] drm_dev_register+0xc7/0x150 [drm]
[  164.459462]  [<ffffffffa014b23f>] drm_get_pci_dev+0x11f/0x1f0 [drm]
[  164.466554]  [<ffffffff816abb81>] ? _raw_spin_unlock_irqrestore+0x51/0x70
[  164.474287]  [<ffffffffa02cf7a6>] i915_pci_probe+0x56/0x60 [i915]
[  164.481185]  [<ffffffff8130a028>] pci_device_probe+0x78/0xf0
[  164.487603]  [<ffffffff813cd495>] driver_probe_device+0x155/0x350
[  164.494505]  [<ffffffff813cd74e>] __driver_attach+0x6e/0xa0
[  164.500826]  [<ffffffff813cd6e0>] ? __device_attach+0x50/0x50
[  164.507333]  [<ffffffff813cb2be>] bus_for_each_dev+0x6e/0xc0
[  164.513752]  [<ffffffff813ccefe>] driver_attach+0x1e/0x20
[  164.519870]  [<ffffffff813cc958>] bus_add_driver+0x138/0x260
[  164.526289]  [<ffffffffa0188000>] ? 0xffffffffa0187fff
[  164.532116]  [<ffffffff813cde78>] driver_register+0x98/0xe0
[  164.538558]  [<ffffffffa0188000>] ? 0xffffffffa0187fff
[  164.544389]  [<ffffffff813087b0>] __pci_register_driver+0x60/0x70
[  164.551336]  [<ffffffffa014b37d>] drm_pci_init+0x6d/0x120 [drm]
[  164.558040]  [<ffffffffa0188000>] ? 0xffffffffa0187fff
[  164.563928]  [<ffffffffa018806a>] i915_init+0x6a/0x6c [i915]
[  164.570363]  [<ffffffff810002da>] do_one_initcall+0xaa/0x160
[  164.576783]  [<ffffffff8103b140>] ? set_memory_nx+0x40/0x50
[  164.583100]  [<ffffffff810ce7f5>] load_module+0x1fb5/0x2550
[  164.589410]  [<ffffffff810caab0>] ? store_uevent+0x40/0x40
[  164.595628]  [<ffffffff810cee7d>] SyS_init_module+0xed/0x100
[  164.602048]  [<ffffffff816b3c52>] system_call_fastpath+0x16/0x1b

v2: simplify the loop further (Chris)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reported-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65652
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74161Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>