UBI: fix add_fastmap() to use the vid_hdr passed in argument

add_fastmap() is passed a ubi_vid_hdr pointer in argument, but is
referencing the global vidh pointer.
Even if this is correct from a functional point of view (vidh and vid_hdr
point to the same object), it is confusing.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>