replace `nvme_scan(NULL, &ctx)` with `nvme_create_global_ctx`

no need for this here:

```c
	if (nvme_scan(NULL, &ctx))
		return -1;
```

just do a 

```c
	struct nvme_global_ctx *ctx =
		nvme_create_global_ctx(NULL, DEFAULT_LOGLEVEL);
```
