Simplify/fix dnode_move() for dn_zfetch

Previous code tried to keep prefetch streams while moving dnode.  But
it was at least not updating per-stream zs_fetchback pointers, causing
use-after-free on next access.  Instead of that I see much easier and
cleaner to just drop old prefetch state and start new from scratch.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #11936
Closes #11998

module/zfs/dnode.c

@@ -754,7 +754,6 @@ dnode_move_impl(dnode_t *odn, dnode_t *ndn)
 	ASSERT(!RW_LOCK_HELD(&odn->dn_struct_rwlock));
 	ASSERT(MUTEX_NOT_HELD(&odn->dn_mtx));
 	ASSERT(MUTEX_NOT_HELD(&odn->dn_dbufs_mtx));
-	ASSERT(!MUTEX_HELD(&odn->dn_zfetch.zf_lock));
 
 	/* Copy fields. */
 	ndn->dn_objset = odn->dn_objset;
@@ -822,19 +821,14 @@ dnode_move_impl(dnode_t *odn, dnode_t *ndn)
 	ndn->dn_newgid = odn->dn_newgid;
 	ndn->dn_newprojid = odn->dn_newprojid;
 	ndn->dn_id_flags = odn->dn_id_flags;
-	dmu_zfetch_init(&ndn->dn_zfetch, NULL);
-	list_move_tail(&ndn->dn_zfetch.zf_stream, &odn->dn_zfetch.zf_stream);
-	ndn->dn_zfetch.zf_dnode = odn->dn_zfetch.zf_dnode;
+	dmu_zfetch_init(&ndn->dn_zfetch, ndn);
 
 	/*
 	 * Update back pointers. Updating the handle fixes the back pointer of
 	 * every descendant dbuf as well as the bonus dbuf.
 	 */
 	ASSERT(ndn->dn_handle->dnh_dnode == odn);
 	ndn->dn_handle->dnh_dnode = ndn;
-	if (ndn->dn_zfetch.zf_dnode == odn) {
-		ndn->dn_zfetch.zf_dnode = ndn;
-	}
 
 	/*
 	 * Invalidate the original dnode by clearing all of its back pointers.