drm/amdgpu: fix a race in kfd_mem_export_dmabuf()

Using drm_gem_prime_handle_to_fd() to set dmabuf up and insert it into
descriptor table, only to have it looked up by file descriptor and
remove it from descriptor table is not just too convoluted - it's
racy; another thread might have modified the descriptor table while
we'd been going through that song and dance.

Switch kfd_mem_export_dmabuf() to using drm_gem_prime_handle_to_dmabuf()
and leave the descriptor table alone...
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c

@@ -25,7 +25,6 @@
 #include <linux/pagemap.h>
 #include <linux/sched/mm.h>
 #include <linux/sched/task.h>
-#include <linux/fdtable.h>
 #include <drm/ttm/ttm_tt.h>
 
 #include <drm/drm_exec.h>
@@ -818,18 +817,13 @@ static int kfd_mem_export_dmabuf(struct kgd_mem *mem)
 	if (!mem->dmabuf) {
 		struct amdgpu_device *bo_adev;
 		struct dma_buf *dmabuf;
-		int r, fd;
 
 		bo_adev = amdgpu_ttm_adev(mem->bo->tbo.bdev);
-		r = drm_gem_prime_handle_to_fd(&bo_adev->ddev, bo_adev->kfd.client.file,
+		dmabuf = drm_gem_prime_handle_to_dmabuf(&bo_adev->ddev, bo_adev->kfd.client.file,
 					       mem->gem_handle,
 			mem->alloc_flags & KFD_IOC_ALLOC_MEM_FLAGS_WRITABLE ?
-					       DRM_RDWR : 0, &fd);
-		if (r)
-			return r;
-		dmabuf = dma_buf_get(fd);
-		close_fd(fd);
-		if (WARN_ON_ONCE(IS_ERR(dmabuf)))
+					       DRM_RDWR : 0);
+		if (IS_ERR(dmabuf))
 			return PTR_ERR(dmabuf);
 		mem->dmabuf = dmabuf;
 	}