Rename the UBI_IOCVOLATTBLK and UBI_IOCVOLDETBLK to UBI_IOCVOLCRBLK and
UBI_IOCVOLRMBLK, because we do not use terms "attach" and "detach" for the R/O
block devices on top of UBI volumes. Instead, we use terms "create" and
"remove". This patch also amends the related commentaries.