bcachefs: Improve bch2_btree_update_start()

bch2_btree_update_start() is now responsible for taking gc_lock and
upgrading the iterator to lock parent nodes - greatly simplifying error
handling and all of the callers.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_gc.c

@@ -1336,11 +1336,10 @@ static void bch2_coalesce_nodes(struct bch_fs *c, struct btree_iter *iter,
 		return;
 	}
 
-	as = bch2_btree_update_start(iter->trans, iter->btree_id,
+	as = bch2_btree_update_start(iter, old_nodes[0]->c.level,
 			btree_update_reserve_required(c, parent) + nr_old_nodes,
 			BTREE_INSERT_NOFAIL|
-			BTREE_INSERT_USE_RESERVE,
-			NULL);
+			BTREE_INSERT_USE_RESERVE);
 	if (IS_ERR(as)) {
 		trace_btree_gc_coalesce_fail(c,
 				BTREE_GC_COALESCE_FAIL_RESERVE_GET);

fs/bcachefs/btree_update_interior.h

@@ -48,6 +48,7 @@ struct btree_update {
 	} mode;
 
 	unsigned			nodes_written:1;
+	unsigned			took_gc_lock:1;
 
 	enum btree_id			btree_id;
 
@@ -120,8 +121,7 @@ struct btree *__bch2_btree_node_alloc_replacement(struct btree_update *,
 
 void bch2_btree_update_done(struct btree_update *);
 struct btree_update *
-bch2_btree_update_start(struct btree_trans *, enum btree_id, unsigned,
-			unsigned, struct closure *);
+bch2_btree_update_start(struct btree_iter *, unsigned, unsigned, unsigned);
 
 void bch2_btree_interior_update_will_free_node(struct btree_update *,
 					       struct btree *);