ubi: Ensure no fastmap flush after uif_close

On detach UBI attempts to update fastmap after closing user interfaces
but at this point UBI volumes have already been free()'ed and fastmap
can no longer access these data structures.

Signed-off-by: Martin Townsend <mtownsend1973@gmail.com>
Cc: hs@denx.de
Cc: kmpark@infradead.org
Cc: richard@sigma-star.at