bcachefs: add check for btree id against max in try read node

Add check for read node's btree_id against BTREE_ID_NR_MAX in
try_read_btree_node to prevent triggering EBUG_ON condition in
bch2_btree_id_root[1].

[1] https://syzkaller.appspot.com/bug?extid=cf7b2215b5d70600ec00

Reported-by: syzbot+cf7b2215b5d70600ec00@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=cf7b2215b5d70600ec00
Fixes: 4409b808 ("bcachefs: Repair pass for scanning for btree nodes")
Signed-off-by: Piotr Zalewski <pZ010001011111@proton.me>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_node_scan.c

@@ -171,6 +171,9 @@ static void try_read_btree_node(struct find_btree_nodes *f, struct bch_dev *ca,
 	if (BTREE_NODE_LEVEL(bn) >= BTREE_MAX_DEPTH)
 		return;
 
+	if (BTREE_NODE_ID(bn) >= BTREE_ID_NR_MAX)
+		return;
+
 	rcu_read_lock();
 	struct found_btree_node n = {
 		.btree_id	= BTREE_NODE_ID(bn),