btrfs: export helpers for subvolume name/id resolution

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>