block: fix deadlock between bd_link_disk_holder and partition scan

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

block: fix deadlock between bd_link_disk_holder and partition scan

'open_mutex' of gendisk is used to protect open/close block devices. But

in bd_link_disk_holder(), it is used to protect the creation of symlink

between holding disk and slave bdev, which introduces some issues.

When bd_link_disk_holder() is called, the driver is usually in the process

of initialization/modification and may suspend submitting io. At this

time, any io hold 'open_mutex', such as scanning partitions, can cause

deadlocks. For example, in raid:

T1 T2

bdev_open_by_dev

lock open_mutex [1]

...

efi_partition

...

md_submit_bio

md_ioctl mddev_syspend

-> suspend all io

md_add_new_disk

bind_rdev_to_array

bd_link_disk_holder

try lock open_mutex [2]

md_handle_request

-> wait mddev_resume

T1 scan partition, T2 add a new device to raid. T1 waits for T2 to resume

mddev, but T2 waits for open_mutex held by T1. Deadlock occurs.

Fix it by introducing a local mutex 'blk_holder_mutex' to replace

'open_mutex'.