mds: cleanup ambiguous slave update when master mds fails

mds: cleanup ambiguous slave update when master mds fails

When auth mds of rename source dentry fails, slave updates in witness
mds become ambiguous. Witnesses need to ask the master if they should
rollback the updates. This type of rollback is special, corresponding
MDRequest struct need to be preserved after rollback. If the master
mds also fails, slave updates in witness mds are no longer special.
Corresponding MDRequest struct need to be cleanup after rollback.

see commit e62e48bb for more information.

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