drm/amdgpu: use vma_lookup() in amdgpu_ttm_tt_get_user_pages()

Use vma_lookup() to find the VMA at a specific address.  As vma_lookup()
will return NULL if the address is not within any VMA, the start address
no longer needs to be validated.

Link: https://lkml.kernel.org/r/20210521174745.2219620-14-Liam.Howlett@Oracle.comSigned-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Laurent Dufour <ldufour@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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

@@ -709,8 +709,8 @@ int amdgpu_ttm_tt_get_user_pages(struct amdgpu_bo *bo, struct page **pages)
 	}
 
 	mmap_read_lock(mm);
-	vma = find_vma(mm, start);
-	if (unlikely(!vma || start < vma->vm_start)) {
+	vma = vma_lookup(mm, start);
+	if (unlikely(!vma)) {
 		r = -EFAULT;
 		goto out_unlock;
 	}