ubi: Fix race condition between ubi volume creation and udev

commit a51a0c8d upstream.

Similar to commit 714fb87e ("ubi: Fix race condition between ubi
device creation and udev"), we should make the volume active before
registering it.

Signed-off-by: Clay McClure <clay@daemons.net>
Signed-off-by: Richard Weinberger <richard@nod.at>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>