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>