bcachefs: Fix uninitialized data in bch2_gc_btree()

Running the filesystem under valgrind exposed a path where the max_stale
variable in bch2_gc_btree() might not be initialized before use in a
rare case when there are no btree nodes in a transaction.
Signed-off-by: Justin Husted <sigstop@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_gc.c

@@ -218,7 +218,7 @@ static int bch2_gc_btree(struct bch_fs *c, enum btree_id btree_id,
 		: expensive_debug_checks(c)		? 0
 		: !btree_node_type_needs_gc(btree_id)	? 1
 		: 0;
-	u8 max_stale;
+	u8 max_stale = 0;
 	int ret = 0;
 
 	bch2_trans_init(&trans, c, 0, 0);