PCI/DOE: Fix destroy_work_on_stack() race

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

PCI/DOE: Fix destroy_work_on_stack() race

The following debug object splat was observed in testing:

ODEBUG: free active (active state 0) object: 0000000097d23782 object type: work_struct hint: doe_statemachine_work+0x0/0x510
WARNING: CPU: 1 PID: 71 at lib/debugobjects.c:514 debug_print_object+0x7d/0xb0
...
Workqueue: pci 0000:36:00.0 DOE [1 doe_statemachine_work
RIP: 0010:debug_print_object+0x7d/0xb0
...
Call Trace:
? debug_print_object+0x7d/0xb0
? __pfx_doe_statemachine_work+0x10/0x10
debug_object_free.part.0+0x11b/0x150
doe_statemachine_work+0x45e/0x510
process_one_work+0x1d4/0x3c0

This occurs because destroy_work_on_stack() was called after signaling
the completion in the calling thread. This creates a race between
destroy_work_on_stack() and the task->work struct going out of scope in
pci_doe().

Signal the work complete after destroying the work struct. This is safe
because signal_task_complete() is the final thing the work item does and
the workqueue code is careful not to access the work struct after.