UBI: initialize static volumes with vol->used_bytes

I came across a problem which seems to be present since:

commit 941dfb07
UBI: set correct gluebi device size

ubi_create_gluebi() leaves mtd->size = 0 for static volumes. So even
existing static volumes are initialized with a size of 0.

Signed-off-by: Jan Altenberg <jan.altenberg@linutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>