btrfs: move btrfs_find_highest_objectid/btrfs_find_free_objectid to disk-io.c

Those functions are going to be used even after inode cache is removed
so moved them to a more appropriate place.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>