ubi: fastmap: Cancel work upon detach

Ben Hutchings pointed out that 29b7a6fa1e ("ubi: fastmap: Don't flush
fastmap work on detach") does not really fix the problem, it just
reduces the risk to hit the race window where fastmap work races against
free()'ing ubi->volumes[].

The correct approach is making sure that no more fastmap work is in
progress before we free ubi data structures.
So we cancel fastmap work right after the ubi background thread is
stopped.
By setting ubi->thread_enabled to zero we make sure that no further work
tries to wake the thread.

Fixes: 29b7a6fa1e ("ubi: fastmap: Don't flush fastmap work on detach")
Fixes: 74cdaf2400 ("UBI: Fastmap: Fix memory leaks while closing the WL sub-system")
Cc: stable@vger.kernel.org
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Cc: Martin Townsend <mtownsend1973@gmail.com>

Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/build.c

@@ -1091,6 +1091,9 @@ int ubi_detach_mtd_dev(int ubi_num, int anyway)
 	if (ubi->bgt_thread)
 		kthread_stop(ubi->bgt_thread);
 
+#ifdef CONFIG_MTD_UBI_FASTMAP
+	cancel_work_sync(&ubi->fm_work);
+#endif
 	ubi_debugfs_exit_dev(ubi);
 	uif_close(ubi);