Fix for bug #6700

When preparing OSD disks with colocated journals, the intialization process
fails when using dmcrypt. The kernel fails to re-read the partition table after
the storage partition is created because the journal partition is already in use
by dmcrypt. This fix unmaps the journal partition from dmcrypt and allows the
partition table to be read.

Signed-off-by: Stephen F Taylor <steveftaylor@gmail.com>
Merge pull request #1948 from steveftaylor/6700

Fix for bug #6700

Reviewed-by: Alfredo Deza <alfredo.deza@inktank.com>