Use ioctl to get block device size

AMLogic based device uses paths like /dev/block/recovery and the
stock init binary either deletes or does not create mmcblk0p12
which breaks TWRP because TWRP cannot match up the path / name.
The ioctl method is probably more reliable anyway and certainly
should be faster.

Change-Id: I73f981dcec637cdf5b189bdefa00ea15b924b500