devcgroup_css_free() calls dev_exception_clean() without the devcgroup_mutex being locked.

Shutting down a kvm virt was giving me the following trace:

[36280.732764] ------------[ cut here ]------------
[36280.732778] WARNING: at /home/snits/dev/linux/security/device_cgroup.c:172 dev_exception_clean+0xa9/0xc0()
[36280.732782] Hardware name: Studio XPS 8100
[36280.732785] Modules linked in: xt_REDIRECT fuse ebtable_nat ebtables ipt_MASQUERADE iptable_nat nf_nat_ipv4 nf_nat xt_CHECKSUM iptable_mangle bridge stp llc nf_conntrack_ipv4 ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 nf_defrag_ipv4 ip6table_filter it87 hwmon_vid xt_state nf_conntrack ip6_tables snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_intel snd_hda_codec snd_hwdep snd_seq coretemp snd_seq_device crc32c_intel snd_pcm snd_page_alloc snd_timer snd broadcom tg3 serio_raw i7core_edac edac_core ptp pps_core lpc_ich pcspkr mfd_core soundcore microcode i2c_i801 nfsd auth_rpcgss nfs_acl lockd vhost_net sunrpc tun macvtap macvlan kvm_intel kvm uinput binfmt_misc autofs4 usb_storage firewire_ohci firewire_core crc_itu_t radeon drm_kms_helper ttm
[36280.732921] Pid: 933, comm: libvirtd Tainted: G        W    3.8.0-rc3-00307-g4c217de #1
[36280.732922] Call Trace:
[36280.732927]  [<ffffffff81044303>] warn_slowpath_common+0x93/0xc0
[36280.732930]  [<ffffffff8104434a>] warn_slowpath_null+0x1a/0x20
[36280.732932]  [<ffffffff812deaf9>] dev_exception_clean+0xa9/0xc0
[36280.732934]  [<ffffffff812deb2a>] devcgroup_css_free+0x1a/0x30
[36280.732938]  [<ffffffff810ccd76>] cgroup_diput+0x76/0x210
[36280.732941]  [<ffffffff8119eac0>] d_delete+0x120/0x180
[36280.732943]  [<ffffffff81195cff>] vfs_rmdir+0xef/0x130
[36280.732945]  [<ffffffff81195e47>] do_rmdir+0x107/0x1c0
[36280.732949]  [<ffffffff8132d17e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[36280.732951]  [<ffffffff81198646>] sys_rmdir+0x16/0x20
[36280.732954]  [<ffffffff8173bd82>] system_call_fastpath+0x16/0x1b
[36280.732956] ---[ end trace ca39dced899a7d9f ]---

Signed-off-by: Jerry Snitselaar <jerry.snitselaar@oracle.com>
Cc: stable@kernel.org
Signed-off-by: James Morris <james.l.morris@oracle.com>