tools/ceph-objectstore-tool: split filestore directories offline to target hash level

tools/ceph-objectstore-tool: split filestore directories offline to target hash level

Currently ceph-objectstore-tool can only split dirs that already
meet the usual object number criteria. It won't reduce the cost of
online split for those dirs that would meet the criteria online.

This change uses 'arg1' option for apply-layout-settings to specify
the target hash level wants to split to. Then dirs will be split
offline no matters how many objects they have. This also helps
those pools that forgot to do pre-splitting at their creation time.

Signed-off-by: Zhi Zhang <zhangz.david@outlook.com>