chunkfs: move creating of kmem_cache into init, was in fill_super

We destroy this cache object at module exit, so the right way for now is
to allocate object while we init module.

Bug can be reproduced as follow:
$ insmod chunkfs.ko
$ grep chunkfs /proc/slabinfo
$ rmmod chunkfs
Killed

[559475.849315] chunkfs (C) 2007 Valerie Henson <val@nmt.edu>
[559524.399996] BUG: unable to handle kernel NULL pointer dereference at 0000000000000068
[559524.400548] IP: [<ffffffff810f6d2a>] kmem_cache_destroy+0x19/0xe5
[559524.401439] PGD 0
[559524.401589] Oops: 0002 [#1] PREEMPT SMP
[559524.401881] Modules linked in: chunkfs(O-) loop fuse joydev hid_generic usbhid hid microcode ohci_pci ohci_hcd ehci_hcd usbcore snd_pcm snd_page_alloc snd_timer snd psmouse parport_pc parport usb_common soundcore pcspkr processor thermal_sys evdev serio_raw i2c_piix4 i2c_core ac button ext4 crc16 jbd2 mbcache sd_mod sg sr_mod crc_t10dif cdrom crct10dif_common ata_generic ahci libahci ata_piix libata scsi_mod e1000
[559524.403936] CPU: 0 PID: 10432 Comm: rmmod Tainted: G           O 3.13.0-rc3+ #96
[559524.403936] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[559524.403936] task: ffff88007d358050 ti: ffff88007af08000 task.ti: ffff88007af08000
[559524.403936] RIP: 0010:[<ffffffff810f6d2a>]  [<ffffffff810f6d2a>] kmem_cache_destroy+0x19/0xe5
[559524.403936] RSP: 0018:ffff88007af09ed8  EFLAGS: 00010246
[559524.403936] RAX: ffff88007d358050 RBX: 0000000000000000 RCX: ffff88007d358050
[559524.403936] RDX: 0000000000000000 RSI: 0000000000000060 RDI: ffffffff814f96e2
[559524.403936] RBP: ffff88007af09ee0 R08: 0000000000000002 R09: ffff88007d3587a0
[559524.403936] R10: 0000000000000000 R11: 000000000000b821 R12: 0000000000000000
[559524.403936] R13: 0000000000000800 R14: 00007f121bb82090 R15: 0000000000000800
[559524.403936] FS:  00007f121a2f1700(0000) GS:ffff88007fc00000(0000) knlGS:0000000000000000
[559524.403936] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[559524.403936] CR2: 0000000000000068 CR3: 000000007ae60000 CR4: 00000000000006f0
[559524.403936] Stack:
[559524.403936]  ffffffffa0282690 ffff88007af09ef0 ffffffffa02807e0 ffff88007af09f78
[559524.403936]  ffffffff81090c1c 000000007af09f28 ffffffffa0282690 0000000000000800
[559524.403936]  ffff88007af09f04 0073666b6e756863 00007f121bb82090 0000000000000800
[559524.403936] Call Trace:
[559524.403936]  [<ffffffffa02807e0>] exit_chunkfs_fs+0x1c/0x83c [chunkfs]
[559524.403936]  [<ffffffff81090c1c>] SyS_delete_module+0x175/0x1fb
[559524.403936]  [<ffffffff813c7323>] ? do_page_fault+0x9/0xb
[559524.403936]  [<ffffffff813ca0e2>] system_call_fastpath+0x16/0x1b
[559524.403936] Code: 55 48 c7 c7 c0 15 65 81 48 89 e5 e8 c4 bb 2c 00 5d c3 55 48 89 e5 53 48 89 fb e8 48 4b f4 ff 48 c7 c7 c0 15 65 81 e8 10 c2 2c 00 <ff> 4b 68 0f 85 af 00 00 00 48 8b 43 60 48 8b 53 58 48 89 df 48
[559524.403936] RIP  [<ffffffff810f6d2a>] kmem_cache_destroy+0x19/0xe5
[559524.403936]  RSP <ffff88007af09ed8>
[559524.403936] CR2: 0000000000000068
[559524.425530] ---[ end trace 7177eb8445efe174 ]---