UBI: make UBI_IOCVOLCRBLK take a parameter for future usage

In order to allow a future ioctl parameter, such as a creation flag,
we change the UBI_IOCVOLCRBLK so it accepts a struct ubi_blkcreate_req.
For the time being the structure is not in use, but fully reserved.

This ABI change is still possible and harmless, because the ioctl has just
been introduced and there's no userspace program which uses it.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>