DDT: Make ddt_zap_walk() use cursor _by_dnode functions

ddt_zap_walk() already receives dnode as an argument.  We can save
one dnode_hold() per entry by using _by_dnode cursor functions.
zap_cursor_init_noprefetch_by_dnode() is added in the process.

Signed-off-by: Alexander Motin <[email protected]>
Reviewed-by: Rob Norris <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Closes #18888

include/sys/zap.h

@@ -474,6 +474,7 @@ void zap_cursor_fini(zap_cursor_t *zc);
  */
 int zap_cursor_init_noprefetch(zap_cursor_t *zc, objset_t *os,
     uint64_t zapobj);
+int zap_cursor_init_noprefetch_by_dnode(zap_cursor_t *zc, dnode_t *dn);
 
 /*
  * Initialize a zap cursor pointing to the position recorded by

module/zfs/ddt_zap.c

@@ -204,10 +204,9 @@ ddt_zap_walk(dnode_t *dn, uint64_t *walk, ddt_key_t *ddk,
 		 * scrub I/Os for each ZAP block that we read in, so
 		 * reading the ZAP is unlikely to be the bottleneck.
 		 */
-		zap_cursor_init_noprefetch(&zc, dn->dn_objset, dn->dn_object);
+		zap_cursor_init_noprefetch_by_dnode(&zc, dn);
 	} else {
-		zap_cursor_init_serialized(&zc, dn->dn_objset, dn->dn_object,
-		    *walk);
+		zap_cursor_init_serialized_by_dnode(&zc, dn, *walk);
 	}
 	if ((error = zap_cursor_retrieve(&zc, za)) == 0) {
 		uint64_t csize = za->za_num_integers;

module/zfs/zap.c

@@ -1108,6 +1108,12 @@ zap_cursor_init_noprefetch(zap_cursor_t *zc, objset_t *os, uint64_t zapobj)
 	return (zap_cursor_init_impl(zc, os, zapobj, 0, B_FALSE));
 }
 
+int
+zap_cursor_init_noprefetch_by_dnode(zap_cursor_t *zc, dnode_t *dn)
+{
+	return (zap_cursor_init_by_dnode_impl(zc, dn, 0, B_FALSE));
+}
+
 int
 zap_cursor_init_serialized(zap_cursor_t *zc, objset_t *os, uint64_t zapobj,
     uint64_t serialized)