btrfs: Don't generate UUID for non-fs tree

btrfs_create_tree() will unconditionally generate UUID for any root.
So for quota tree and data reloc tree created by kernel, they will have
unique UUIDs.

However UUID in root item is only referred by UUID tree, which only
records UUID for fs trees.  This makes unique UUIDs for quota/data reloc
tree meaningless.

Leave the UUID as zero for non-fs tree, making btrfs-debug-tree output
less confusing.

Reported-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>