Revert "blockdev: Remove the --setbsz (set blocksize) option which has never worked."

This reverts commit b1555acc2f709ac4f3b1e6c686a11cadb7b04f72.

It seems that the option is used by kernel guys to test kernel, so
let's keep the option in the blockdev(8) although it's almost useless
in userspace. All we need is to improve docs to make things more
obvious to end users.

Signed-off-by: Karel Zak <kzak@redhat.com>
blockdev: add note about --setbsz usability

Signed-off-by: Karel Zak <kzak@redhat.com>