mds: properly set COMPLETE flag when merging dirfrags

mds: properly set COMPLETE flag when merging dirfrags

don't keep the COMPLETE flag when merging dirfrags during journal
replay, because it's inconvenience to check if the all dirfrags
under the 'basefrag' are in the cache and complete. One special case
is that newly created dirfrag get fragmented, then the fragment
operation get rolled back. The COMPLETE flag should be preserved in
this case because the dirfrag still doesn't exist on object store.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>