LU-12137 osd-ldiskfs: refactor osd_ios_ROOT_scan

LU-12137 osd-ldiskfs: refactor osd_ios_ROOT_scan

With the change of osd_ios_scan_one() to handle a NULL
dchild->d_inode we can migrate the scrub code after calling
osd_ios_scan_one(). This can help reduce the code indentation
as well as make the code clearer.

Change-Id: Ieb59aa85c8199a1f1a1bee5cbe633e52b8e92a9a
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/35803
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>