mds: fix 'force dirfrags' during journal replay

mds: fix 'force dirfrags' during journal replay

For rename operation, null dentry is first replayed, it detaches
the inode from the FS hierarchy. Then primary dentry is replayed,
it updates the inode and re-attaches the inode to the FS hierarchy.
We may call CInode::force_dirfrag() when updating the inode. But
CInode::force_dirfrag() doesn't work well when inode is detached
from the FS hierarchy because adjusting fragments may also adjust
subtree map. The fix is don't detach the inode when replaying the
null dentry.

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