drm/nouveau/uvmm: clear the dirty flag when unwinding an OP_UNMAP_SPARSE

In the Linux kernel, the following vulnerability has been resolved: drm/nouveau/uvmm: clear the dirty flag when unwinding an OP_UNMAP_SPARSE A successful OP_UNMAP_SPARSE marks its region dirty with nouveau_uvma_region_dirty() and defers the teardown to nouveau_uvmm_bind_job_cleanup(); it does not remove the region from uvmm->region_mt. If a later op in the job fails, the unwind path never clears reg->dirty (set in one place, cleared nowhere) and sets op->reg = NULL, so cleanup skips the teardown. The region is left in the tree with dirty set and its completion never signalled. Later binds over that range then fail permanently -- -ENOENT or -EINVAL from the dirty checks, or an unkillable wait_for_completion() in bind_validate_region() -- for the lifetime of the uvmm. Clear reg->dirty when the unwind reverts the sparse unmap, restoring the region to the state it was found in.