md: make rdev_addable usable for rcu mode

Our testcase trigger panic:

BUG: kernel NULL pointer dereference, address: 00000000000000e0
...
Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 2 UID: 0 PID: 85 Comm: kworker/2:1 Not tainted 6.16.0+ #94
PREEMPT(none)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.16.1-2.fc37 04/01/2014
Workqueue: md_misc md_start_sync
RIP: 0010:rdev_addable+0x4d/0xf0
...
Call Trace:
 <TASK>
 md_start_sync+0x329/0x480
 process_one_work+0x226/0x6d0
 worker_thread+0x19e/0x340
 kthread+0x10f/0x250
 ret_from_fork+0x14d/0x180
 ret_from_fork_asm+0x1a/0x30
 </TASK>
Modules linked in: raid10
CR2: 00000000000000e0
---[ end trace 0000000000000000 ]---
RIP: 0010:rdev_addable+0x4d/0xf0

md_spares_need_change in md_start_sync will call rdev_addable which
protected by rcu_read_lock/rcu_read_unlock. This rcu context will help
protect rdev won't be released, but rdev->mddev will be set to NULL
before we call synchronize_rcu in md_kick_rdev_from_array. Fix this by
using READ_ONCE and check does rdev->mddev still alive.

Fixes: bc08041b32 ("md: suspend array in md_start_sync() if array need reconfiguration")
Fixes: 570b9147de ("md: use RCU lock to protect traversal in md_spares_need_change()")
Signed-off-by: Yang Erkun <yangerkun@huawei.com>
Link: https://lore.kernel.org/linux-raid/20250731114530.776670-1-yangerkun@huawei.com
Signed-off-by: Yu Kuai <yukuai3@huawei.com>

drivers/md/md.c

@@ -9427,6 +9427,12 @@ static bool rdev_is_spare(struct md_rdev *rdev)
 
 static bool rdev_addable(struct md_rdev *rdev)
 {
+	struct mddev *mddev;
+
+	mddev = READ_ONCE(rdev->mddev);
+	if (!mddev)
+		return false;
+
 	/* rdev is already used, don't add it again. */
 	if (test_bit(Candidate, &rdev->flags) || rdev->raid_disk >= 0 ||
 	    test_bit(Faulty, &rdev->flags))
@@ -9437,7 +9443,7 @@ static bool rdev_addable(struct md_rdev *rdev)
 		return true;
 
 	/* Allow to add if array is read-write. */
-	if (md_is_rdwr(rdev->mddev))
+	if (md_is_rdwr(mddev))
 		return true;
 
 	/*