UBI: block: do not use term "attach"

We already use term attach/detach for UBI->MTD relations, let's not use this
for UBI->ubiblock relations to avoid confusion. Just use 'create' and 'remove'
instead. E.g., "create a R/O block device on top of a UBI volume".

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>