[ Upstream commit c0c907a4 ]

The functions will be used outside of export.c and super.c to allow
resolving subvolume name from a given id, eg. for subvolume deletion by
id ioctl.
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ split from the next patch ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>