drm/amdgpu: Refactor amdgpu_gem_va_ioctl for Handling Last Fence Update and Timeline Management v4

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

drm/amdgpu: Refactor amdgpu_gem_va_ioctl for Handling Last Fence Update and Timeline Management v4

This commit simplifies the amdgpu_gem_va_ioctl function, key updates
include:
- Moved the logic for managing the last update fence directly into
amdgpu_gem_va_update_vm.
- Introduced checks for the timeline point to enable conditional
replacement or addition of fences.

v2: Addressed review comments from Christian.
v3: Updated comments (Christian).
v4: The previous version selected the fence too early and did not manage its
reference correctly, which could lead to stale or freed fences being used.
This resulted in refcount underflows and could crash when updating GPU
timelines.
The fence is now chosen only after the VA mapping work is completed, and its
reference is taken safely. After exporting it to the VM timeline syncobj, the
driver always drops its local fence reference, ensuring balanced refcounting
and avoiding use-after-free on dma_fence.

Crash signature:
[ 205.828135] refcount_t: underflow; use-after-free.
[ 205.832963] WARNING: CPU: 30 PID: 7274 at lib/refcount.c:28 refcount_warn_saturate+0xbe/0x110
...
[ 206.074014] Call Trace:
[ 206.076488] <TASK>
[ 206.078608] amdgpu_gem_va_ioctl+0x6ea/0x740 [amdgpu]
[ 206.084040] ? __pfx_amdgpu_gem_va_ioctl+0x10/0x10 [amdgpu]
[ 206.089994] drm_ioctl_kernel+0x86/0xe0 [drm]
[ 206.094415] drm_ioctl+0x26e/0x520 [drm]
[ 206.098424] ? __pfx_amdgpu_gem_va_ioctl+0x10/0x10 [amdgpu]
[ 206.104402] amdgpu_drm_ioctl+0x4b/0x80 [amdgpu]
[ 206.109387] __x64_sys_ioctl+0x96/0xe0
[ 206.113156] do_syscall_64+0x66/0x2d0
...
[ 206.553351] BUG: unable to handle page fault for address: ffffffffc0dfde90
...
[ 206.553378] RIP: 0010:dma_fence_signal_timestamp_locked+0x39/0xe0
...
[ 206.553405] Call Trace:
[ 206.553409] <IRQ>
[ 206.553415] ? __pfx_drm_sched_fence_free_rcu+0x10/0x10 [gpu_sched]
[ 206.553424] dma_fence_signal+0x30/0x60
[ 206.553427] drm_sched_job_done.isra.0+0x123/0x150 [gpu_sched]
[ 206.553434] dma_fence_signal_timestamp_locked+0x6e/0xe0
[ 206.553437] dma_fence_signal+0x30/0x60
[ 206.553441] amdgpu_fence_process+0xd8/0x150 [amdgpu]
[ 206.553854] sdma_v4_0_process_trap_irq+0x97/0xb0 [amdgpu]
[ 206.554353] edac_mce_amd(E) ee1004(E)
[ 206.554270] amdgpu_irq_dispatch+0x150/0x230 [amdgpu]
[ 206.554702] amdgpu_ih_process+0x6a/0x180 [amdgpu]
[ 206.555101] amdgpu_irq_handler+0x23/0x60 [amdgpu]
[ 206.555500] __handle_irq_event_percpu+0x4a/0x1c0
[ 206.555506] handle_irq_event+0x38/0x80
[ 206.555509] handle_edge_irq+0x92/0x1e0
[ 206.555513] __common_interrupt+0x3e/0xb0
[ 206.555519] common_interrupt+0x80/0xa0
[ 206.555525] </IRQ>
[ 206.555527] <TASK>
...
[ 206.555650] RIP: 0010:dma_fence_signal_timestamp_locked+0x39/0xe0
...
[ 206.555667] Kernel panic - not syncing: Fatal exception in interrupt