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

At this point UBI volumes have already been free()'ed and fastmap can no
longer access these data structures.

Reported-by: Martin Townsend <[e-mail]>
Fixes: 74cdaf24004a ("UBI: Fastmap: Fix memory leaks while closing the WL sub-system")
Cc: [e-mail]
Signed-off-by: Richard Weinberger <[e-mail]>

drivers/mtd/ubi/fastmap-wl.c

···
 {
 	int i;
 
-	flush_work(&ubi->fm_work);
 	return_unused_pool_pebs(ubi, &ubi->fm_pool);
 	return_unused_pool_pebs(ubi, &ubi->fm_wl_pool);