Properly release key in spa_keystore_dsl_key_hold_dd()

Since dsl_crypto_key_open() references the key, 0d23f5e2e4 should
have called dsl_crypto_key_rele() to drop it first instead of
calling dsl_crypto_key_free() directly. The final result should
actually be the same, but without triggering dck_holds assertion.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16567