drm/amdkfd: svm range restore work deadlock when process exit

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

drm/amdkfd: svm range restore work deadlock when process exit

kfd_process_notifier_release flush svm_range_restore_work
which calls svm_range_list_lock_and_flush_work to flush deferred_list
work, but if deferred_list work mmput release the last user, it will
call exit_mmap -> notifier_release, it is deadlock with below backtrace.

Move flush svm_range_restore_work to kfd_process_wq_release to avoid
deadlock. Then svm_range_restore_work take task->mm ref to avoid mm is
gone while validating and mapping ranges to GPU.

Workqueue: events svm_range_deferred_list_work [amdgpu]
Call Trace:
wait_for_completion+0x94/0x100
__flush_work+0x12a/0x1e0
__cancel_work_timer+0x10e/0x190
cancel_delayed_work_sync+0x13/0x20
kfd_process_notifier_release+0x98/0x2a0 [amdgpu]
__mmu_notifier_release+0x74/0x1f0
exit_mmap+0x170/0x200
mmput+0x5d/0x130
svm_range_deferred_list_work+0x104/0x230 [amdgpu]
process_one_work+0x220/0x3c0