blkdiscard: don't report "zero range" at the end

blkdiscard: don't report "zero range" at the end

We want to address the case that we have printed the last useful
stats line already within the loop. Avoiding an additional line
"Discarded 0 bytes ..." at the end.

Note there is a behavior change now for the edge cases "-v -l 0" and
"-v -o blksize" where we don't print any stats line anymore. But actually
it's correct, we never make any BLKDISCARD syscall with zero range.
Perhaps we should return error in these cases to help people who always
want to parse stats output on success.

CC: Federico Simoncelli <fsimonce@redhat.com>
Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>