md: Don't set mddev private to NULL in raid0 pers->free

In the Linux kernel, the following vulnerability has been resolved:

md: Don't set mddev private to NULL in raid0 pers->free

In normal stop process, it does like this:
do_md_stop
|
__md_stop (pers->free(); mddev->private=NULL)
|
md_free (free mddev)
__md_stop sets mddev->private to NULL after pers->free. The raid device
will be stopped and mddev memory is free. But in reshape, it doesn't
free the mddev and mddev will still be used in new raid.

In reshape, it first sets mddev->private to new_pers and then runs
old_pers->free(). Now raid0 sets mddev->private to NULL in raid0_free.
The new raid can't work anymore. It will panic when dereference
mddev->private because of NULL pointer dereference.

It can panic like this:
[63010.814972] kernel BUG at drivers/md/raid10.c:928!
[63010.819778] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
[63010.825011] CPU: 3 PID: 44437 Comm: md0_resync Kdump: loaded Not tainted 5.14.0-86.el9.x86_64 #1
[63010.833789] Hardware name: Dell Inc. PowerEdge R6415/07YXFK, BIOS 1.15.0 09/11/2020
[63010.841440] RIP: 0010:raise_barrier+0x161/0x170 [raid10]
[63010.865508] RSP: 0018:ffffc312408bbc10 EFLAGS: 00010246
[63010.870734] RAX: 0000000000000000 RBX: ffffa00bf7d39800 RCX: 0000000000000000
[63010.877866] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffa00bf7d39800
[63010.884999] RBP: 0000000000000000 R08: fffffa4945e74400 R09: 0000000000000000
[63010.892132] R10: ffffa00eed02f798 R11: 0000000000000000 R12: ffffa00bbc435200
[63010.899266] R13: ffffa00bf7d39800 R14: 0000000000000400 R15: 0000000000000003
[63010.906399] FS: 0000000000000000(0000) GS:ffffa00eed000000(0000) knlGS:0000000000000000
[63010.914485] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[63010.920229] CR2: 00007f5cfbe99828 CR3: 0000000105efe000 CR4: 00000000003506e0
[63010.927363] Call Trace:
[63010.929822] ? bio_reset+0xe/0x40
[63010.933144] ? raid10_alloc_init_r10buf+0x60/0xa0 [raid10]
[63010.938629] raid10_sync_request+0x756/0x1610 [raid10]
[63010.943770] md_do_sync.cold+0x3e4/0x94c
[63010.947698] md_thread+0xab/0x160
[63010.951024] ? md_write_inc+0x50/0x50
[63010.954688] kthread+0x149/0x170
[63010.957923] ? set_kthread_struct+0x40/0x40
[63010.962107] ret_from_fork+0x22/0x30

Removing the code that sets mddev->private to NULL in raid0 can fix
problem.