dm: fix a race condition in retrieve_deps

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

dm: fix a race condition in retrieve_deps

There's a race condition in the multipath target when retrieve_deps

races with multipath_message calling dm_get_device and dm_put_device.

retrieve_deps walks the list of open devices without holding any lock

but multipath may add or remove devices to the list while it is

running. The end result may be memory corruption or use-after-free

memory access.

See this description of a UAF with multipath_message():

https://listman.redhat.com/archives/dm-devel/2022-October/052373.html

Fix this bug by introducing a new rw semaphore "devices_lock". We grab

devices_lock for read in retrieve_deps and we grab it for write in

dm_get_device and dm_put_device.