bcachefs: Fix btree node scan when unknown btree IDs are present

btree_root entries for unknown btree IDs are created during recovery,
before reading those btree roots.

But btree_node_scan may find btree nodes with unknown btree IDs when we
haven't seen roots for those btrees.

Reported-by:  <syzbot+1f202d4da221ec6ebf8e@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>