When we change group_thread_cnt from sysfs entry, it can OOPS.

The kernel messages are:
[  135.299021] BUG: unable to handle kernel NULL pointer dereference at           (null)
[  135.299073] IP: [<ffffffff815188ab>] handle_active_stripes+0x32b/0x440
[  135.299107] PGD 0
[  135.299122] Oops: 0000 [#1] SMP
[  135.299144] Modules linked in: netconsole e1000e ptp pps_core
[  135.299188] CPU: 3 PID: 2225 Comm: md0_raid5 Not tainted 3.12.0+ #24
[  135.299214] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./To be filled by O.E.M., BIOS 080015  11/09/2011
[  135.299255] task: ffff8800b9638f80 ti: ffff8800b77a4000 task.ti: ffff8800b77a4000
[  135.299283] RIP: 0010:[<ffffffff815188ab>]  [<ffffffff815188ab>] handle_active_stripes+0x32b/0x440
[  135.299323] RSP: 0018:ffff8800b77a5c48  EFLAGS: 00010002
[  135.299344] RAX: ffff880037bb5c70 RBX: 0000000000000000 RCX: 0000000000000008
[  135.299371] RDX: ffff880037bb5cb8 RSI: 0000000000000001 RDI: ffff880037bb5c00
[  135.299398] RBP: ffff8800b77a5d08 R08: 0000000000000001 R09: 0000000000000000
[  135.299425] R10: ffff8800b77a5c98 R11: 00000000ffffffff R12: ffff880037bb5c00
[  135.299452] R13: 0000000000000000 R14: 0000000000000000 R15: ffff880037bb5c70
[  135.299479] FS:  0000000000000000(0000) GS:ffff88013fd80000(0000) knlGS:0000000000000000
[  135.299510] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  135.299532] CR2: 0000000000000000 CR3: 0000000001c0b000 CR4: 00000000000407e0
[  135.299559] Stack:
[  135.299570]  ffff8800b77a5c88 ffffffff8107383e ffff8800b77a5c88 ffff880037a64300
[  135.299611]  000000000000ec08 ffff880037bb5cb8 ffff8800b77a5c98 ffffffffffffffd8
[  135.299654]  000000000000ec08 ffff880037bb5c60 ffff8800b77a5c98 ffff8800b77a5c98
[  135.299696] Call Trace:
[  135.299711]  [<ffffffff8107383e>] ? __wake_up+0x4e/0x70
[  135.299733]  [<ffffffff81518f88>] raid5d+0x4c8/0x680
[  135.299756]  [<ffffffff817174ed>] ? schedule_timeout+0x15d/0x1f0
[  135.299781]  [<ffffffff81524c9f>] md_thread+0x11f/0x170
[  135.299804]  [<ffffffff81069cd0>] ? wake_up_bit+0x40/0x40
[  135.299826]  [<ffffffff81524b80>] ? md_rdev_init+0x110/0x110
[  135.299850]  [<ffffffff81069656>] kthread+0xc6/0xd0
[  135.299871]  [<ffffffff81069590>] ? kthread_freezable_should_stop+0x70/0x70
[  135.299899]  [<ffffffff81722ffc>] ret_from_fork+0x7c/0xb0
[  135.299923]  [<ffffffff81069590>] ? kthread_freezable_should_stop+0x70/0x70
[  135.299951] Code: ff ff ff 0f 84 d7 fe ff ff e9 5c fe ff ff 66 90 41 8b b4 24 d8 01 00 00 45 31 ed 85 f6 0f 8e 7b fd ff ff 49 8b 9c 24 d0 01 00 00 <48> 3b 1b 49 89 dd 0f 85 67 fd ff ff 48 8d 43 28 31 d2 eb 17 90
[  135.300005] RIP  [<ffffffff815188ab>] handle_active_stripes+0x32b/0x440
[  135.300005]  RSP <ffff8800b77a5c48>
[  135.300005] CR2: 0000000000000000
[  135.300005] ---[ end trace 504854e5bb7562ed ]---
[  135.300005] Kernel panic - not syncing: Fatal exception

This is because raid5d() can be running when the multi-thread
resources are changed via system. We see need to provide locking.

mddev->device_lock is suitable, but we cannot simple call
alloc_thread_groups under this lock as we cannot allocate memory
while holding a spinlock.
So change alloc_thread_groups() to allocate and return the data
structures, then raid5_store_group_thread_cnt() can take the lock
while updating the pointers to the data structures.

This fixes a bug introduced in 3.12 and so is suitable for the 3.12.x
stable series.

Fixes: b721420e
Cc: stable@vger.kernel.org (3.12)
Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Shaohua Li <shli@kernel.org>