tasks: add mds_auto_repair

New CephFS tests for MDS's auto repair functions. (So far the only
test case is verify/repair backtrace on fetch dirfrag)

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Hmmm, strictly this is racy but it'll be replaced by a call to the asok flush command soon so I guess I don't mind.