netfilter: IDLETIMER: fix lockdep warning

netfilter: IDLETIMER: fix lockdep warning

Dynamically allocated sysfs attributes should be initialized with
sysfs_attr_init() otherwise lockdep will be angry with us:

[   45.468653] BUG: key ffffffc030fad4e0 not in .data!
[   45.468655] ------------[ cut here ]------------
[   45.468666] WARNING: CPU: 0 PID: 1176 at /mnt/host/source/src/third_party/kernel/v3.18/kernel/locking/lockdep.c:2991 lockdep_init_map+0x12c/0x490()
[   45.468672] DEBUG_LOCKS_WARN_ON(1)
[   45.468672] CPU: 0 PID: 1176 Comm: iptables Tainted: G     U  W 3.18.0 #43
[   45.468674] Hardware name: XXX
[   45.468675] Call trace:
[   45.468680] [<ffffffc0002072b4>] dump_backtrace+0x0/0x10c
[   45.468683] [<ffffffc0002073d0>] show_stack+0x10/0x1c
[   45.468688] [<ffffffc000a86cd4>] dump_stack+0x74/0x94
[   45.468692] [<ffffffc000217ae0>] warn_slowpath_common+0x84/0xb0
[   45.468694] [<ffffffc000217b84>] warn_slowpath_fmt+0x4c/0x58
[   45.468697] [<ffffffc0002530a4>] lockdep_init_map+0x128/0x490
[   45.468701] [<ffffffc000367ef0>] __kernfs_create_file+0x80/0xe4
[   45.468704] [<ffffffc00036862c>] sysfs_add_file_mode_ns+0x104/0x170
[   45.468706] [<ffffffc00036870c>] sysfs_create_file_ns+0x58/0x64
[   45.468711] [<ffffffc000930430>] idletimer_tg_checkentry+0x14c/0x324
[   45.468714] [<ffffffc00092a728>] xt_check_target+0x170/0x198
[   45.468717] [<ffffffc000993efc>] check_target+0x58/0x6c
[   45.468720] [<ffffffc000994c64>] translate_table+0x30c/0x424
[   45.468723] [<ffffffc00099529c>] do_ipt_set_ctl+0x144/0x1d0
[   45.468728] [<ffffffc0009079f0>] nf_setsockopt+0x50/0x60
[   45.468732] [<ffffffc000946870>] ip_setsockopt+0x8c/0xb4
[   45.468735] [<ffffffc0009661c0>] raw_setsockopt+0x10/0x50
[   45.468739] [<ffffffc0008c1550>] sock_common_setsockopt+0x14/0x20
[   45.468742] [<ffffffc0008bd190>] SyS_setsockopt+0x88/0xb8
[   45.468744] ---[ end trace 41d156354d18c039 ]---

Signed-off-by: Dmitry Torokhov <dtor@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>