bcachefs: Handle errors in bch2_trans_mark_update()

It's not actually the case that iterators are always checked here -
__bch2_trans_commit() checks for that after running triggers.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -293,8 +293,10 @@ int bch2_alloc_read(struct bch_fs *c, struct journal_keys *journal_keys)
 {
 	int ret;
 
+	down_read(&c->gc_lock);
 	ret = bch2_btree_and_journal_walk(c, journal_keys, BTREE_ID_alloc,
 					  NULL, bch2_alloc_read_fn);
+	up_read(&c->gc_lock);
 	if (ret) {
 		bch_err(c, "error reading alloc info: %i", ret);
 		return ret;

fs/bcachefs/buckets.c

@@ -1836,10 +1836,11 @@ int bch2_trans_mark_update(struct btree_trans *trans,
 		return 0;
 
 	if (!btree_node_type_is_extents(iter->btree_id)) {
-		/* iterators should be uptodate, shouldn't get errors here: */
 		if (btree_iter_type(iter) != BTREE_ITER_CACHED) {
 			old = bch2_btree_iter_peek_slot(iter);
-			BUG_ON(bkey_err(old));
+			ret = bkey_err(old);
+			if (ret)
+				return ret;
 		} else {
 			struct bkey_cached *ck = (void *) iter->l[0].b;