[media] rc: add locking to fix register/show race

[media] rc: add locking to fix register/show race

When device_add is called in rc_register_device, the rc sysfs nodes show
up, and there's a window in which ir-keytable can be launched via udev
and trigger a show_protocols call, which runs without various rc_dev
fields filled in yet. Add some locking around registration and
store/show_protocols to prevent that from happening.

The problem manifests thusly:

[64692.957872] BUG: unable to handle kernel NULL pointer dereference at 0000000000000090
[64692.957878] IP: [<ffffffffa036a4c1>] show_protocols+0x47/0xf1 [rc_core]
[64692.957890] PGD 19cfc7067 PUD 19cfc6067 PMD 0
[64692.957894] Oops: 0000 [#1] SMP
[64692.957897] last sysfs file: /sys/devices/pci0000:00/0000:00:03.1/usb3/3-1/3-1:1.0/rc/rc2/protocols
[64692.957902] CPU 3
[64692.957903] Modules linked in: redrat3(+) ir_lirc_codec lirc_dev ir_sony_decoder ir_jvc_decoder ir_rc6_decoder ir_rc5_decoder rc_hauppauge ir_nec
_decoder rc_core ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_filter ip6_tables snd_emu10k1_synth snd_emux_synth snd_seq_virmidi snd_seq_mi
di_event snd_seq_midi_emul snd_emu10k1 snd_rawmidi snd_ac97_codec ac97_bus snd_seq snd_pcm snd_seq_device snd_timer snd_page_alloc snd_util_mem pcsp
kr tg3 snd_hwdep emu10k1_gp snd amd64_edac_mod gameport edac_core soundcore edac_mce_amd k8temp shpchp i2c_piix4 lm63 e100 mii uinput ipv6 raid0 rai
d1 ata_generic firewire_ohci pata_acpi firewire_core crc_itu_t sata_svw pata_serverworks floppy radeon ttm drm_kms_helper drm i2c_algo_bit i2c_core
[last unloaded: redrat3]
[64692.957949] [64692.957952] Pid: 12265, comm: ir-keytable Tainted: G   M    W   2.6.39-rc6+ #2 empty empty/TYAN Thunder K8HM S3892
[64692.957957] RIP: 0010:[<ffffffffa036a4c1>]  [<ffffffffa036a4c1>] show_protocols+0x47/0xf1 [rc_core]
[64692.957962] RSP: 0018:ffff880194509e38  EFLAGS: 00010202
[64692.957964] RAX: 0000000000000000 RBX: ffffffffa036d1e0 RCX: ffffffffa036a47a
[64692.957966] RDX: ffff88019a84d000 RSI: ffffffffa036d1e0 RDI: ffff88019cf2f3f0
[64692.957969] RBP: ffff880194509e68 R08: 0000000000000002 R09: 0000000000000000
[64692.957971] R10: 0000000000000002 R11: 0000000000001617 R12: ffff88019a84d000
[64692.957973] R13: 0000000000001000 R14: ffff8801944d2e38 R15: ffff88019ce5f190
[64692.957976] FS:  00007f0a30c9a720(0000) GS:ffff88019fc00000(0000) knlGS:0000000000000000
[64692.957979] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[64692.957981] CR2: 0000000000000090 CR3: 000000019a8e0000 CR4: 00000000000006e0
[64692.957983] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[64692.957986] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[64692.957989] Process ir-keytable (pid: 12265, threadinfo ffff880194508000, task ffff88019a9fc720)
[64692.957991] Stack:
[64692.957992]  0000000000000002 ffffffffa036d1e0 ffff880194509f58 0000000000001000
[64692.957997]  ffff8801944d2e38 ffff88019ce5f190 ffff880194509e98 ffffffff8131484b
[64692.958001]  ffffffff8118e923 ffffffff810e9b2f ffff880194509e98 ffff8801944d2e18
[64692.958005] Call Trace:
[64692.958014]  [<ffffffff8131484b>] dev_attr_show+0x27/0x4e
[64692.958014]  [<ffffffff8118e923>] ? sysfs_read_file+0x94/0x172
[64692.958014]  [<ffffffff810e9b2f>] ? __get_free_pages+0x16/0x52
[64692.958014]  [<ffffffff8118e94c>] sysfs_read_file+0xbd/0x172
[64692.958014]  [<ffffffff8113205e>] vfs_read+0xac/0xf3
[64692.958014]  [<ffffffff8113347b>] ? fget_light+0x3a/0xa1
[64692.958014]  [<ffffffff811320f2>] sys_read+0x4d/0x74
[64692.958014]  [<ffffffff814c19c2>] system_call_fastpath+0x16/0x1b

Its a bit difficult to reproduce, but I'm fairly confident this has
fixed the problem.

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>