btrfs: lift uuid_mutex to callers of btrfs_parse_early_options

Prepartory work to fix race between mount and device scan.

btrfs_parse_early_options calls the device scan from mount and we'll
need to let mount completely manage the critical section.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -890,6 +890,8 @@ static int btrfs_parse_early_options(const char *options, fmode_t flags,
 	char *device_name, *opts, *orig, *p;
 	int error = 0;
 
+	lockdep_assert_held(&uuid_mutex);
+
 	if (!options)
 		return 0;
 
@@ -915,10 +917,8 @@ static int btrfs_parse_early_options(const char *options, fmode_t flags,
 				error = -ENOMEM;
 				goto out;
 			}
-			mutex_lock(&uuid_mutex);
 			error = btrfs_scan_one_device(device_name,
 					flags, holder, fs_devices);
-			mutex_unlock(&uuid_mutex);
 			kfree(device_name);
 			if (error)
 				goto out;
@@ -1526,8 +1526,10 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
 	if (!(flags & SB_RDONLY))
 		mode |= FMODE_WRITE;
 
+	mutex_lock(&uuid_mutex);
 	error = btrfs_parse_early_options(data, mode, fs_type,
 					  &fs_devices);
+	mutex_unlock(&uuid_mutex);
 	if (error) {
 		return ERR_PTR(error);
 	}