drm/amdgpu: keep the eviction fence until the last handle is closed

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

drm/amdgpu: keep the eviction fence until the last handle is closed

amdgpu_gem_object_close() detaches the user queue eviction fence from
the BO before it checks bo_va->ref_count, so closing any handle of a BO
removes the fence, even when the file still has another handle to the
BO and the bo_va with its GPU mappings stays. The BO can then be evicted
without suspending the user queues of the file, which keep accessing it
through mappings that are only marked for update.

Seen on navi31 with a SDMA user queue: a BO opened twice in the same
file (GEM_FLINK + GEM_OPEN) loses its eviction fence when the second
handle is closed (traced attach/detach of the fence):

attach bo=ffff8c78a6c84c00
GEM_CLOSE handle=7
detach bo=ffff8c78a6c84c00

while handle 6 and the mapping are still in use.

Only detach the fence when the last reference to the bo_va goes away,
matching amdgpu_gem_object_open(), which attaches it only when the
bo_va is created. With this the fence stays until the file is closed.

(cherry picked from commit 8387982f03d4026b7dc11b0b5e3118298eee4d6a)