BRT should return EOPNOTSUPP

Return the more descriptive EOPNOTSUPP instead of EXDEV when the
storage pool doesn't support block cloning.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Signed-off-by: Kay Pedersen <mail@mkwg.de>
Closes #15097