dax: Fix dax_mapping_release() use after free

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

dax: Fix dax_mapping_release() use after free

A CONFIG_DEBUG_KOBJECT_RELEASE test of removing a device-dax region

provider (like modprobe -r dax_hmem) yields:

kobject: 'mapping0' (ffff93eb460e8800): kobject_release, parent 0000000000000000 (delayed 2000)

[..]

DEBUG_LOCKS_WARN_ON(1)

WARNING: CPU: 23 PID: 282 at kernel/locking/lockdep.c:232 __lock_acquire+0x9fc/0x2260

[..]

RIP: 0010:__lock_acquire+0x9fc/0x2260

[..]

Call Trace:

<TASK>

[..]

lock_acquire+0xd4/0x2c0

? ida_free+0x62/0x130

_raw_spin_lock_irqsave+0x47/0x70

? ida_free+0x62/0x130

ida_free+0x62/0x130

dax_mapping_release+0x1f/0x30

device_release+0x36/0x90

kobject_delayed_cleanup+0x46/0x150

Due to attempting ida_free() on an ida object that has already been

freed. Devices typically only hold a reference on their parent while

registered. If a child needs a parent object to complete its release it

needs to hold a reference that it drops from its release callback.

Arrange for a dax_mapping to pin its parent dev_dax instance until

dax_mapping_release().