btrfs: Rename find_oldest_super_backup to init_backup_root_slot

The old name name was an awful misnomer because it didn't really find
the oldest super backup per-se but rather its slot. For example if we
have:

slot0: gen - 2
slot1: gen - 1
slot2: gen
slot3: empty

init_backup_root_slot will return slot3 and not slot0.

The new name is more appropriate since the function doesn't care whether
there is a valid backup in the returned slot or not.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -1815,13 +1815,11 @@ static int find_newest_super_backup(struct btrfs_fs_info *info)
 	return -EINVAL;
 }
 
-
 /*
- * find the oldest backup so we know where to store new entries
- * in the backup array.  This will set the backup_root_index
- * field in the fs_info struct
+ * Initialize backup_root_index with the next available slot, where subsequent
+ * transaction commit will store the backup root
  */
-static void find_oldest_super_backup(struct btrfs_fs_info *info)
+static void init_backup_root_slot(struct btrfs_fs_info *info)
 {
 	int newest_index;
 
@@ -2935,7 +2933,7 @@ int __cold open_ctree(struct super_block *sb,
 	 * run through our array of backup supers and setup
 	 * our ring pointer to the oldest one
 	 */
-	find_oldest_super_backup(fs_info);
+	init_backup_root_slot(fs_info);
 
 	/*
 	 * In the long term, we'll store the compression type in the super