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 <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>