ddt: remove struct names and forward declarations

Things get confused when there's more than one name for a thing.

Note that we don't do this for ddt_object_t, ddt_histogram_t and
ddt_stat_t because they're part of the public ZFS interface.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Closes #15887

include/sys/ddt_impl.h

@@ -36,7 +36,7 @@ extern "C" {
 /*
  * Ops vector to access a specific DDT object type.
  */
-typedef struct ddt_ops {
+typedef struct {
 	char ddt_op_name[32];
 	int (*ddt_op_create)(objset_t *os, uint64_t *object, dmu_tx_t *tx,
 	    boolean_t prehash);