btrfs: simplify cleanup after error in btrfs_create_tree

Since leaf is already NULL, and no other branch will go to fail_unlock,
the fail_unlock label is useless and can be removed

Signed-off-by: Peng Hao <flyingpeng@tencent.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>