bcachefs: Fix missing call to bch2_fs_allocator_background_exit()

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super.c

@@ -544,6 +544,7 @@ static void __bch2_fs_free(struct bch_fs *c)
 
 	bch2_find_btree_nodes_exit(&c->found_btree_nodes);
 	bch2_free_pending_node_rewrites(c);
+	bch2_fs_allocator_background_exit(c);
 	bch2_fs_sb_errors_exit(c);
 	bch2_fs_counters_exit(c);
 	bch2_fs_snapshots_exit(c);