mds: do not add null dentry to frozen dir
mds: do not add null dentry to frozen dir

Otherwise we get confused when we unfreeze:

mds/mdstypes.h:1193: FAILED assert(ref_set.count(by) > 0)
 1: (MDSCacheObject::bad_put(int)+0x3e) [0x79acfc]
 2: (MDSCacheObject::put(int)+0x5a) [0x79e332]
 3: (CDentry::finish_export()+0x52) [0x8874ba]
 4: (Migrator::finish_export_dir(CDir*, std::list<Context*, std::allocator<Context*> >&, utime_t)+0x1a4) [0x8751fc]
 5: (Migrator::finish_export_dir(CDir*, std::list<Context*, std::allocator<Context*> >&, utime_t)+0x243) [0x87529b]

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph