gpu: nvgpu: fix bad error code in gmmu map

__nvgpu_gmmu_map() does not currently have a mechanism for returning an
error code if something goes wrong; it just returns zero on error, or a
nonzero address when mapping was successful. Change it to return 0
instead of -ENOMEM if the creation of nvgpu_sgt from mem fails. Out of
memory is the only meaningful failure case anyway, and this -ENOMEM
would just look like a (huge) memory address.

Change-Id: If176e165b1f9a8fa84a1bdf3080c9059a193081f
Signed-off-by: Konsta Holtta <kholtta@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1812491
Reviewed-by: svc-misra-checker <svc-misra-checker@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Alex Waterman <alexw@nvidia.com>
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/common/mm/gmmu.c

@@ -80,7 +80,7 @@ static u64 __nvgpu_gmmu_map(struct vm_gk20a *vm,
 	struct nvgpu_sgt *sgt = nvgpu_sgt_create_from_mem(g, mem);
 
 	if (!sgt) {
-		return -ENOMEM;
+		return 0;
 	}
 
 	/*