Add a ZAP API to move a ZAP cursor to a given key.

Index: zfs+chaos4/lib/libzfscommon/include/sys/zap.h
===================================================================
--- zfs+chaos4.orig/lib/libzfscommon/include/sys/zap.h
+++ zfs+chaos4/lib/libzfscommon/include/sys/zap.h
@@ -302,6 +302,11 @@ void zap_cursor_advance(zap_cursor_t *zc
 uint64_t zap_cursor_serialize(zap_cursor_t *zc);
 
 /*
+ * Advance the cursor to the attribute having the key.
+ */
+int zap_cursor_move_to_key(zap_cursor_t *zc, const char *name, matchtype_t mt);
+
+/*
  * Initialize a zap cursor pointing to the position recorded by
  * zap_cursor_serialize (in the "serialized" argument).  You can also
  * use a "serialized" argument of 0 to start at the beginning of the
Index: zfs+chaos4/lib/libzfscommon/include/sys/zap_impl.h
===================================================================
--- zfs+chaos4.orig/lib/libzfscommon/include/sys/zap_impl.h
+++ zfs+chaos4/lib/libzfscommon/include/sys/zap_impl.h
@@ -210,6 +210,7 @@ int fzap_add_cd(zap_name_t *zn,
     uint64_t integer_size, uint64_t num_integers,
     const void *val, uint32_t cd, dmu_tx_t *tx);
 void fzap_upgrade(zap_t *zap, dmu_tx_t *tx);
+int fzap_cursor_move_to_key(zap_cursor_t *zc, zap_name_t *zn);
 
 #ifdef	__cplusplus
 }
Index: zfs+chaos4/lib/libzpool/zap.c
===================================================================
--- zfs+chaos4.orig/lib/libzpool/zap.c
+++ zfs+chaos4/lib/libzpool/zap.c
@@ -1029,6 +1029,30 @@ zap_stats_ptrtbl(zap_t *zap, uint64_t *t
 	}
 }
 
+int fzap_cursor_move_to_key(zap_cursor_t *zc, zap_name_t *zn)
+{
+	int err;
+	zap_leaf_t *l;
+	zap_entry_handle_t zeh;
+	uint64_t hash;
+
+	if (zn->zn_name_orij && strlen(zn->zn_name_orij) > ZAP_MAXNAMELEN)
+		return (E2BIG);
+
+	err = zap_deref_leaf(zc->zc_zap, zn->zn_hash, NULL, RW_READER, &l);
+	if (err != 0)
+		return (err);
+
+	err = zap_leaf_lookup(l, zn, &zeh);
+	if (err != 0)
+		return (err);
+
+	zc->zc_leaf = l;
+	zc->zc_hash = zeh.zeh_hash;
+	zc->zc_cd = zeh.zeh_cd;
+	return 0;
+}
+
 void
 fzap_get_stats(zap_t *zap, zap_stats_t *zs)
 {
Index: zfs+chaos4/lib/libzpool/zap_micro.c
===================================================================
--- zfs+chaos4.orig/lib/libzpool/zap_micro.c
+++ zfs+chaos4/lib/libzpool/zap_micro.c
@@ -1045,6 +1045,45 @@ zap_cursor_advance(zap_cursor_t *zc)
 	}
 }
 
+int zap_cursor_move_to_key(zap_cursor_t *zc, const char *name, matchtype_t mt)
+{
+	int err = 0;
+	mzap_ent_t *mze;
+	zap_name_t *zn;
+
+	if (zc->zc_zap == NULL) {
+		err = zap_lockdir(zc->zc_objset, zc->zc_zapobj, NULL,
+				RW_READER, TRUE, FALSE, &zc->zc_zap);
+		if (err)
+			return (err);
+	} else {
+		rw_enter(&zc->zc_zap->zap_rwlock, RW_READER);
+	}
+
+	zn = zap_name_alloc(zc->zc_zap, name, mt);
+	if (zn == NULL) {
+		rw_exit(&zc->zc_zap->zap_rwlock);
+		return (ENOTSUP);
+	}
+
+	if (!zc->zc_zap->zap_ismicro) {
+		err = fzap_cursor_move_to_key(zc, zn);
+	} else {
+		mze = mze_find(zn);
+		if (mze == NULL) {
+			err = (ENOENT);
+			goto out;
+		}
+		zc->zc_hash = mze->mze_hash;
+		zc->zc_cd = mze->mze_phys.mze_cd;
+	}
+
+out:
+	zap_name_free(zn);
+	rw_exit(&zc->zc_zap->zap_rwlock);
+	return (err);
+}
+
 int
 zap_get_stats(objset_t *os, uint64_t zapobj, zap_stats_t *zs)
 {