bcachefs: Clean up bch2_extent_can_insert

It was using an internal btree node iterator interface, when
bch2_btree_iter_peek_slot() sufficed. We were hitting a null ptr deref
that looked like it was from the iterator not being uptodate - this will
also fix that.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/extent_update.c

@@ -192,18 +192,13 @@ bch2_extent_can_insert(struct btree_trans *trans,
 		       struct btree_iter *iter,
 		       struct bkey_i *insert)
 {
-	struct btree_iter_level *l = &iter->l[0];
-	struct btree_node_iter node_iter = l->iter;
-	struct bkey_packed *_k;
 	struct bkey_s_c k;
-	struct bkey unpacked;
-	int sectors;
+	int ret, sectors;
 
-	_k = bch2_btree_node_iter_peek(&node_iter, l->b);
-	if (!_k)
-		return BTREE_INSERT_OK;
-
-	k = bkey_disassemble(l->b, _k, &unpacked);
+	k = bch2_btree_iter_peek_slot(iter);
+	ret = bkey_err(k);
+	if (ret)
+		return ret;
 
 	/* Check if we're splitting a compressed extent: */