accel/amdxdna: Fix potential amdxdna_umap lifetime race

In the Linux kernel, the following vulnerability has been resolved: accel/amdxdna: Fix potential amdxdna_umap lifetime race amdxdna_umap_release() calls the blocking mmu_interval_notifier_remove() before removing the object from abo->mem.umap_list. If aie2_populate_range() runs concurrently, it may obtain a reference to an amdxdna_umap that is being released, leading to a potential use-after-free. Use kref_get_unless_zero() in aie2_populate_range() when acquiring a reference. If the reference count has already dropped to zero, release is in progress and the entry is skipped.