mds: avoid infinite loop in MDLog::submit_entry()

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: fix NULL pointer dereference in Server::handle_client_rename

Inode's parent dentry can be NULL, which causes segment fault in
Locker::acquire_locks

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