librbd: For format 2, don't forget set objectcache max objects.

librbd: For format 2, don't forget set objectcache max objects.

For format 2, before create object_cache, it call init_layout and set
the max object for object cache.
In the later ictx_refresh, it only set for format1.
Now move set logic after creating object cache.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>