Export minimal zfs_refcount interfaces

Lustre makes light use of the zfs_refcount interfaces which
isn't a problem when using a non-debug build of OpenZFS. However,
when debugging is enabled the required symbols are not exported.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12613