UBI: fix freeing ubi->vtbl while unloading

ubi->vtbl is allocated using vmalloc() in vtbl.c empty_create_lvol(),
but it is freed in build.c with kfree()

Signed-off-by: Vinit Agnihotri <vinit.agnihotri@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>