README.md: Some smoothing

Some small corrections/clarifications to the text + paragraphs
formatting.

Signed-off-by: Borislav Petkov <bp@suse.de>
badblocks.sh: Print some initial help info

... in case user calls without sufficient number of args on cmdline.

Signed-off-by: Borislav Petkov <bp@suse.de>
badblocks.sh: Check for valid blocksizes

The filesystem can have only distinct blocksizes, so we should
be checking for these explictly.

Signed-off-by: Hannes Reinecke <hare@suse.de>