mds: verify backtrace when fetching dirfrag

Fixes: #9557
Signed-off-by: Yan, Zheng <zyan@redhat.com>
mds: store backtrace for straydir

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #2769 from ceph/wip-9557

mds: verify backtrace when fetching dirfrag

Reviewed-by: John Spray <john.spray@redhat.com>