bcachefs: mount: fix null deref with null devname

 - Fix null deref on mount when given a null device name.
 - Move the dev_name checks to return EINVAL when it is invalid.
Signed-off-by: Dan Robertson <dan@dlrobertson.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs.c

@@ -1325,9 +1325,6 @@ static char **split_devs(const char *_dev_name, unsigned *nr)
 	char *dev_name = NULL, **devs = NULL, *s;
 	size_t i, nr_devs = 0;
 
-	if (strlen(_dev_name) == 0)
-		return NULL;
-
 	dev_name = kstrdup(_dev_name, GFP_KERNEL);
 	if (!dev_name)
 		return NULL;
@@ -1503,6 +1500,9 @@ static struct dentry *bch2_mount(struct file_system_type *fs_type,
 	if (ret)
 		return ERR_PTR(ret);
 
+	if (!dev_name || strlen(dev_name) == 0)
+		return ERR_PTR(-EINVAL);
+
 	devs = split_devs(dev_name, &nr_devs);
 	if (!devs)
 		return ERR_PTR(-ENOMEM);