nvmet: seset ns->file when open fails

Reset the ns->file value to NULL also in the error case in
nvmet_file_ns_enable().

The ns->file variable points either to file object or contains the
error code after the filp_open() call. This can lead to following
problem:

When the user first setups an invalid file backend and tries to enable
the ns, it will fail. Then the user switches over to a bdev backend
and enables successfully the ns. The first received I/O will crash the
system because the IO backend is chosen based on the ns->file value:

static u16 nvmet_parse_io_cmd(struct nvmet_req *req)
{
	[...]

	if (req->ns->file)
		return nvmet_file_parse_io_cmd(req);

	return nvmet_bdev_parse_io_cmd(req);
}
Reported-by: Enzo Matsumiya <ematsumiya@suse.com>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/io-cmd-file.c

@@ -49,9 +49,11 @@ int nvmet_file_ns_enable(struct nvmet_ns *ns)
 
 	ns->file = filp_open(ns->device_path, flags, 0);
 	if (IS_ERR(ns->file)) {
-		pr_err("failed to open file %s: (%ld)\n",
-				ns->device_path, PTR_ERR(ns->file));
-		return PTR_ERR(ns->file);
+		ret = PTR_ERR(ns->file);
+		pr_err("failed to open file %s: (%d)\n",
+			ns->device_path, ret);
+		ns->file = NULL;
+		return ret;
 	}
 
 	ret = nvmet_file_ns_revalidate(ns);