block: partition: initialize percpuref before sending out KOBJ_ADD

The initialization of partition's percpu_ref should have been done before
sending out KOBJ_ADD uevent, which may cause userspace to read partition
table. So the uninitialized percpu_ref may be accessed in data path.

This patch fixes this issue reported by Naveen.

Reported-by: Naveen Kaje <nkaje@codeaurora.org>
Tested-by: Naveen Kaje <nkaje@codeaurora.org>
Fixes: 6c71013e(block: partition: convert percpu ref)
Cc: <stable@vger.kernel.org> # v4.3+
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>