BugLink: http://bugs.launchpad.net/bugs/1546439

The initialization of partition's percpu_ref should have been done before
sending out KOBJ_ADD, which may cause userspace to read partition table.

This patch should fix this issue.
Reported-by: Naveen Kaje <nkaje@codeaurora.org>
Fixes: 6c71013e(block: partition: convert percpu ref)
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>