bcachefs: Fix broken btree_path lock invariants in next_node()

This fixes btree locking assert pops users were seeing during evacuate:

koverstreet/bcachefs#878

May 09 22:45:02 sharon kernel: bcachefs (68116e25-fa2d-4c6f-86c7-e8b431d792ae):   bch2_btree_insert_node(): node not locked at level 1
May 09 22:45:02 sharon kernel:   bch2_btree_node_rewrite [bcachefs]: watermark=btree no_check_rw alloc l=0-1 mode=none nodes_written=0 cl.remaining=2 journal_seq=0
May 09 22:45:02 sharon kernel:   path: idx   1 ref 1:0   S B btree=alloc level=0 pos 0:3699637:0 0:3698012:1-0:3699637:0 bch2_move_btree.isra.0+0x1db/0x490 [bcachefs] uptodate 0 locks_want 2
May 09 22:45:02 sharon kernel:     l=0 locks intent seq 4 node ffff8bd700c93600
May 09 22:45:02 sharon kernel:     l=1 locks unlocked seq 1712 node ffff8bd6fd5e7a00
May 09 22:45:02 sharon kernel:     l=2 locks unlocked seq 2295 node ffff8bd6cc725400
May 09 22:45:02 sharon kernel:     l=3 locks unlocked seq 0 node 0000000000000000

Evacuate walks btree nodes with bch2_btree_iter_next_node() and rewrites
them, bch2_btree_update_start() upgrades the path to take intent locks
as far as it needs to.

But next_node() does low level unlock/relock calls on individual nodes,
and didn't handle the case where a path is supposed to be holding
multiple intent locks. If a path has locks_want > 1, it needs to be
either holding locks on all the btree nodes (at each level) requested,
or none of them.

Fix this with a bch2_btree_path_downgrade().

Signed-off-by: Kent Overstreet <[email protected]>

Author: Kent Overstreet

fs/bcachefs/btree_iter.c

@@ -1971,6 +1971,12 @@ struct btree *bch2_btree_iter_next_node(struct btree_trans *trans, struct btree_
 		return NULL;
 	}
 
+	/*
+	 * We don't correctly handle nodes with extra intent locks here:
+	 * downgrade so we don't violate locking invariants
+	 */
+	bch2_btree_path_downgrade(trans, path);
+
 	if (!bch2_btree_node_relock(trans, path, path->level + 1)) {
 		__bch2_btree_path_unlock(trans, path);
 		path->l[path->level].b		= ERR_PTR(-BCH_ERR_no_btree_node_relock);