Inform users that the data block size can't be any arbitrary number,
i.e. its value must be between 64KB and 1GB.  Also, it should be a
multiple of 64KB.
Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>