The read_locks are a simple counter to track locking balance and used to
assert tree locks.  Add helpers to make it conditionally work only in
DEBUG builds.  Will be used in followup patches.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>