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_c...