ubi: fastmap: Don't flush fastmap work on detach

BugLink: http://bugs.launchpad.net/bugs/1768429

commit 29b7a6fa upstream.

At this point UBI volumes have already been free()'ed and fastmap can no
longer access these data structures.
Reported-by: Martin Townsend <mtownsend1973@gmail.com>
Fixes: 74cdaf24 ("UBI: Fastmap: Fix memory leaks while closing the WL sub-system")
Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/mtd/ubi/fastmap-wl.c

@@ -360,7 +360,6 @@ static void ubi_fastmap_close(struct ubi_device *ubi)
 {
 	int i;
 
-	flush_work(&ubi->fm_work);
 	return_unused_pool_pebs(ubi, &ubi->fm_pool);
 	return_unused_pool_pebs(ubi, &ubi->fm_wl_pool);