cxl/port: Hold port reference until decoder release

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

cxl/port: Hold port reference until decoder release

KASAN + DEBUG_KOBJECT_RELEASE reports a potential use-after-free in
cxl_decoder_release() where it goes to reference its parent, a cxl_port,
to free its id back to port->decoder_ida.

BUG: KASAN: use-after-free in to_cxl_port+0x18/0x90 [cxl_core]
Read of size 8 at addr ffff888119270908 by task kworker/35:2/379

CPU: 35 PID: 379 Comm: kworker/35:2 Tainted: G OE 5.17.0-rc2+ #198
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
Workqueue: events kobject_delayed_cleanup
Call Trace:
<TASK>
dump_stack_lvl+0x59/0x73
print_address_description.constprop.0+0x1f/0x150
? to_cxl_port+0x18/0x90 [cxl_core]
kasan_report.cold+0x83/0xdf
? to_cxl_port+0x18/0x90 [cxl_core]
to_cxl_port+0x18/0x90 [cxl_core]
cxl_decoder_release+0x2a/0x60 [cxl_core]
device_release+0x5f/0x100
kobject_cleanup+0x80/0x1c0

The device core only guarantees parent lifetime until all children are
unregistered. If a child needs a parent to complete its ->release()
callback that child needs to hold a reference to extend the lifetime of
the parent.