drm/amdkfd: Flush the process wq before creating a kfd_process

There is a race condition when re-creating a kfd_process for a process.
This has been observed when a process under the debugger executes
exec(3).  In this scenario:
- The process executes exec.
 - This will eventually release the process's mm, which will cause the
   kfd_process object associated with the process to be freed
   (kfd_process_free_notifier decrements the reference count to the
   kfd_process to 0).  This causes kfd_process_ref_release to enqueue
   kfd_process_wq_release to the kfd_process_wq.
- The debugger receives the PTRACE_EVENT_EXEC notification, and tries to
  re-enable AMDGPU traps (KFD_IOC_DBG_TRAP_ENABLE).
 - When handling this request, KFD tries to re-create a kfd_process.
   This eventually calls kfd_create_process and kobject_init_and_add.

At this point the call to kobject_init_and_add can fail because the
old kfd_process.kobj has not been freed yet by kfd_process_wq_release.

This patch proposes to avoid this race by making sure to drain
kfd_process_wq before creating a new kfd_process object.  This way, we
know that any cleanup task is done executing when we reach
kobject_init_and_add.

Signed-off-by: Lancelot SIX <lancelot.six@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>