Btrfs-progs: only enforce a maximum size if we specify one

Btrfs-progs: only enforce a maximum size if we specify one

My patch

04609add88ef8428d725de6ef60f46a3ff0dbc8e

introduced a regression where if you mkfs'ed a group of disks with different
sizes it limited the disks to the size of the first one that is specified.
This was not the intent of my patch, I only want it to limit the size based
on the -b option, so I've reworked the code to pass in a max block count and
that fixes the issue.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>