netfilter: nf_log: fix sleeping function called from invalid context

Fix regression introduced by 17625274 "netfilter: sysctl support of
logger choice":

BUG: sleeping function called from invalid context at /mnt/s390test/linux-2.6-tip/arch/s390/include/asm/uaccess.h:234
in_atomic(): 1, irqs_disabled(): 0, pid: 3245, name: sysctl
CPU: 1 Not tainted 2.6.30-rc8-tipjun10-02053-g39ae214 #1
Process sysctl (pid: 3245, task: 000000007f675da0, ksp: 000000007eb17cf0)
0000000000000000 000000007eb17be8 0000000000000002 0000000000000000
       000000007eb17c88 000000007eb17c00 000000007eb17c00 0000000000048156
       00000000003e2de8 000000007f676118 000000007eb17f10 0000000000000000
       0000000000000000 000000007eb17be8 000000000000000d 000000007eb17c58
       00000000003e2050 000000000001635c 000000007eb17be8 000000007eb17c30
Call Trace:
(<00000000000162e6> show_trace+0x13a/0x148)
 <00000000000349ea> __might_sleep+0x13a/0x164
 <0000000000050300> proc_dostring+0x134/0x22c
 <0000000000312b70> nf_log_proc_dostring+0xfc/0x188
 <0000000000136f5e> proc_sys_call_handler+0xf6/0x118
 <0000000000136fda> proc_sys_read+0x26/0x34
 <00000000000d6e9c> vfs_read+0xac/0x158
 <00000000000d703e> SyS_read+0x56/0x88
 <0000000000027f42> sysc_noemu+0x10/0x16

Use the nf_log_mutex instead of RCU to fix this.

Reported-and-tested-by: Maran Pakkirisamy <maranpsamy@in.ibm.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>