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Issue description: 

The command "[root@serverf ~]# cephadm shell --mount /home/admin/rbd-import" will mount all of rbd-import into /mnt

With that, the command to import the diff is incorrect.

[ceph: root@serverf /]# rbd import-diff /mnt/rbd-import/export-diff.dat rbd/test

should be:

[ceph: root@serverf /]# rbd import-diff /mnt/export-diff.dat rbd/test

Steps to reproduce:

Workaround:

Expected result: