CVE-2025-68373

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

md: avoid repeated calls to del_gendisk

There is a uaf problem which is found by case 23rdev-lifetime:

Oops: general protection fault, probably for non-canonical address 0xdead000000000122
RIP: 0010:bdi_unregister+0x4b/0x170
Call Trace:
 <TASK>
 __del_gendisk+0x356/0x3e0
 mddev_unlock+0x351/0x360
 rdev_attr_store+0x217/0x280
 kernfs_fop_write_iter+0x14a/0x210
 vfs_write+0x29e/0x550
 ksys_write+0x74/0xf0
 do_syscall_64+0xbb/0x380
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff5250a177e

The sequence is:
1. rdev remove path gets reconfig_mutex
2. rdev remove path release reconfig_mutex in mddev_unlock
3. md stop calls do_md_stop and sets MD_DELETED
4. rdev remove path calls del_gendisk because MD_DELETED is set
5. md stop path release reconfig_mutex and calls del_gendisk again

So there is a race condition we should resolve. This patch adds a
flag MD_DO_DELETE to avoid the race condition.