btrfs: WARN_ON_ONCE() in our leak detection code

fstests looks for WARN_ON's in dmesg.  Add WARN_ON_ONCE() to our leak
detection code (enabled only in debug builds) so that fstests will fail
if these things trip at all.  This will allow us to easily catch
problems with our reference counting that may otherwise go unnoticed.

Reviewed-by: Neal Gompa <neal@gompa.dev>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>