reorganise most of the NetBSD portion of nvkm_mem_dtor().

when nvkm_mem_new_host() is called via the in-kernel ioctl method,
we copy the supplied dmamap, use it's dm_nsegs value for allocation
of "mem->dma", and assume it remains valid until we're done.

when this path is taken "mem->mem" remains NULL so all the code in
nvkm_mem_dtor() is ignored, and the "mem->dma" is leaked.  this is
one leak seen in PR#56826.  as "dmamap->dm_nsegs" can become invalid
before the dtor call, store the value in "mem->nseg" for use in the
dtor, and convert the dtor to free "mem->dma" if "mem->dma" is set.

additionally, destroy the dmamap created in the non-ioctl path of
nvkm_mem_new_host().  this is another leak seen in PR#56826.

with both of these fixes my "kmem-04096" pool does not grow rapidly
while using "mpv -vo gpu".  infact, once i loaded the relevant file
into memory, this pool remains stable after at least one minute of


video playback.
Index: external/bsd/drm2/dist/drm/nouveau/nvkm/subdev/mmu/nouveau_nvkm_subdev_mmu_mem.c
===================================================================
RCS file: /cvsroot/src/sys/external/bsd/drm2/dist/drm/nouveau/nvkm/subdev/mmu/nouveau_nvkm_subdev_mmu_mem.c,v
retrieving revision 1.7
diff -p -u -r1.7 nouveau_nvkm_subdev_mmu_mem.c
--- external/bsd/drm2/dist/drm/nouveau/nvkm/subdev/mmu/nouveau_nvkm_subdev_mmu_mem.c	19 Dec 2021 11:06:44 -0000	1.7
+++ external/bsd/drm2/dist/drm/nouveau/nvkm/subdev/mmu/nouveau_nvkm_subdev_mmu_mem.c	31 May 2022 04:23:35 -0000
@@ -97,16 +97,21 @@ static void *
 nvkm_mem_dtor(struct nvkm_memory *memory)
 {
 	struct nvkm_mem *mem = nvkm_mem(memory);
-	if (mem->mem) {
 #ifdef __NetBSD__
+	if (mem->dma) {
+		kmem_free(mem->dma, mem->nseg * sizeof(mem->dma[0]));
+	}
+	if (mem->mem) {
 		struct nvkm_device *device = mem->mmu->subdev.device;
 		bus_dma_tag_t dmat = device->func->dma_tag(device);
-		kmem_free(mem->dma,
-		    mem->dmamap->dm_nsegs * sizeof(mem->dma[0]));
+
 		bus_dmamap_unload(dmat, mem->dmamap);
 		bus_dmamem_free(dmat, mem->mem, mem->nseg);
+		bus_dmamap_destroy(dmat, mem->dmamap);
 		kmem_free(mem->mem, mem->pages * sizeof(mem->mem[0]));
+	}
 #else
+	if (mem->mem) {
 		while (mem->pages--) {
 			dma_unmap_page(mem->mmu->subdev.device->dev,
 				       mem->dma[mem->pages], PAGE_SIZE,
@@ -115,8 +120,8 @@ nvkm_mem_dtor(struct nvkm_memory *memory
 		}
 		kvfree(mem->dma);
 		kvfree(mem->mem);
-#endif
 	}
+#endif
 	return mem;
 }
 
@@ -219,6 +224,9 @@ nvkm_mem_new_host(struct nvkm_mmu *mmu, 
 		if (args->v0.dma) {
 			nvkm_memory_ctor(&nvkm_mem_dma, &mem->memory);
 			mem->dmamap = args->v0.dma;
+#ifdef __NetBSD__
+			mem->nseg = mem->dmamap->dm_nsegs;
+#endif
 			mem->dma = kmem_zalloc(mem->dmamap->dm_nsegs *
 			    sizeof(mem->dma[0]), KM_SLEEP);
 			for (unsigned i = 0; i < mem->dmamap->dm_nsegs; i++) {