ddt: compare keys, not entries

We're about to have different kinds of things that we'll compare on key,
so generalise this function to support that.

(It actually worked fine because of the way the casts work out, but it
requires the key to be at the start of the object so the cast through
ddt_entry_t works, and even then it reads strangely for anything that's
not a ddt_entry_t).

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

cmd/zdb/zdb.c

@@ -7121,6 +7121,7 @@ dump_block_stats(spa_t *spa)
 }
 
 typedef struct zdb_ddt_entry {
+	/* key must be first for ddt_key_compare */
 	ddt_key_t	zdde_key;
 	uint64_t	zdde_ref_blocks;
 	uint64_t	zdde_ref_lsize;
@@ -7181,7 +7182,7 @@ dump_simulated_ddt(spa_t *spa)
 	ddt_histogram_t ddh_total = {{{0}}};
 	ddt_stat_t dds_total = {0};
 
-	avl_create(&t, ddt_entry_compare,
+	avl_create(&t, ddt_key_compare,
 	    sizeof (zdb_ddt_entry_t), offsetof(zdb_ddt_entry_t, zdde_node));
 
 	spa_config_enter(spa, SCL_CONFIG, FTAG, RW_READER);