netfilter: nf_log: fix sleeping function called from invalid context in seq_show()

[  171.925285] BUG: sleeping function called from invalid context at kernel/mutex.c:280
[  171.925296] in_atomic(): 1, irqs_disabled(): 0, pid: 671, name: grep
[  171.925306] 2 locks held by grep/671:
[  171.925312]  #0:  (&p->lock){+.+.+.}, at: [<c10b8acd>] seq_read+0x25/0x36c
[  171.925340]  #1:  (rcu_read_lock){.+.+..}, at: [<c1391dac>] seq_start+0x0/0x44
[  171.925372] Pid: 671, comm: grep Not tainted 2.6.31.6-4-netbook #3
[  171.925380] Call Trace:
[  171.925398]  [<c105104e>] ? __debug_show_held_locks+0x1e/0x20
[  171.925414]  [<c10264ac>] __might_sleep+0xfb/0x102
[  171.925430]  [<c1461521>] mutex_lock_nested+0x1c/0x2ad
[  171.925444]  [<c1391c9e>] seq_show+0x74/0x127
[  171.925456]  [<c10b8c5c>] seq_read+0x1b4/0x36c
[  171.925469]  [<c10b8aa8>] ? seq_read+0x0/0x36c
[  171.925483]  [<c10d5c8e>] proc_reg_read+0x60/0x74
[  171.925496]  [<c10d5c2e>] ? proc_reg_read+0x0/0x74
[  171.925510]  [<c10a4468>] vfs_read+0x87/0x110
[  171.925523]  [<c10a458a>] sys_read+0x3b/0x60
[  171.925538]  [<c1002a49>] syscall_call+0x7/0xb

Fix it by replacing RCU with nf_log_mutex.

Reported-by: "Yin, Kangkai" <kangkai.yin@intel.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>