Fix journal partition creation

With OSD sharing data and journal, the previous code created the
journal partiton from the end of the device. A uint32_t is
used in sgdisk to get the last sector, with large HD, uint32_t
is too small.
The journal partition will be created backwards from the
sector 1849032670 leaving space before and after it. The data
partition will use whichever of these spaces is greater.
The remaining will not be used.

This patch creates the journal partition from the start as a workaround.

Signed-off-by: Alexandre Marangone <alexandre.marangone@inktank.com>