mds: let Locker::acquire_locks()'s caller choose locking order

mds: let Locker::acquire_locks()'s caller choose locking order

This patch makes Locker::acquire_locks() lock objects in the order
specified by its caller. Locker::acquire_locks() only rearranges locks
in the same object (relieve of remembering the order). This patch is
preparation for 'lock object in top-down order'.

Besides, this patch allows MDRequest to lock objects step by step. For
example: call Locker::acquire_locks() to lock a dentry. After the dentry
is locked, call Locker::acquire_locks() to lock inode that is linked by
the dentry.

Locking object step by step introduces a problem. MDRequest may needs to
auth pin extra objects after taking same locks. If any object can not be
auth pinned, MDRequest needs to drop all locks before going to wait. For
slave auth pin request, this patch make slave mds send a notification
back to master mds if the auth pin request is blocked. The master mds
drops locks when receiving the notification.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>