bcachefs: Set BTREE_NODE_SEQ() correctly in merge path

BTREE_NODE_SEQ() is supposed to give us a time ordering of btree nodes
on disk, so that we can tell which btree node is newer if we ever have
to scan the entire device to find btree nodes.

The btree node merge path wasn't setting it correctly on the new node -
oops.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_update_interior.c

@@ -1697,6 +1697,10 @@ int __bch2_foreground_maybe_merge(struct btree_trans *trans,
 	n = bch2_btree_node_alloc(as, b->c.level);
 	bch2_btree_update_add_new_node(as, n);
 
+	SET_BTREE_NODE_SEQ(n->data,
+			   max(BTREE_NODE_SEQ(b->data),
+			       BTREE_NODE_SEQ(m->data)) + 1);
+
 	btree_set_min(n, prev->data->min_key);
 	btree_set_max(n, next->data->max_key);
 	n->data->format		= new_f;