mds: fix check for merging/spliting dirfrag

check actual number of items instead of number of cached items

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1586 from ceph/wip-dirfrag

mds: fix check for merging/spliting dirfrag

Reviewed-by: Sage Weil <sage@inktank.com>