ceph: extract entity name from device id

Previously, the "name" in the new device syntax "<name>@<fsid>.<fsname>"
was ignored because (presumably) tests were done using mount.ceph which
also passed the entity name using "-o name=foo". If mounting is done
without the mount.ceph helper, the new device id syntax fails to set
the name properly.

Cc: stable@vger.kernel.org
Link: https://tracker.ceph.com/issues/68516


Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>