blkdiscard: fix underflow when offset is greater than device size

blkdiscard: fix underflow when offset is greater than device size

If offset (range[0]) is greater than device size (blksize), the variable 'end'
will be greater than blksize, and range[1] (length) will be recalculated.
The underflow happens when subtracting range[0] (offset) from blksize, thus
range[1] will be the result of an underflow. The bug leads to unwanted behavior
from the program, where range[1] is likely to be a high number and then will
discard a considerable amount of blocks from the device. The fix consists of
exitting the program with an error message when the condition stated above is
true. Spotted while auditing the code.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>