bcachefs: Fix shift overflow in btree_lost_data()

Reported-by: syzbot+29f65db1a5fe427b5c56@syzkaller.appspotmail.com
Fixes: 55936afe ("bcachefs: Flag btrees with missing data")
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs_format.h

@@ -1555,6 +1555,12 @@ enum btree_id {
 	BTREE_ID_NR
 };
 
+/*
+ * Maximum number of btrees that we will _ever_ have under the current scheme,
+ * where we refer to them with bitfields
+ */
+#define BTREE_ID_NR_MAX		64
+
 static inline bool btree_id_is_alloc(enum btree_id id)
 {
 	switch (id) {

fs/bcachefs/recovery.c

@@ -35,6 +35,9 @@
 
 void bch2_btree_lost_data(struct bch_fs *c, enum btree_id btree)
 {
+	if (btree >= BTREE_ID_NR_MAX)
+		return;
+
 	u64 b = BIT_ULL(btree);
 
 	if (!(c->sb.btrees_lost_data & b)) {