mgr/volumes: Fix idempotent subvolume rm

mgr/volumes: Fix idempotent subvolume rm

The subvolume deletion of a subvolume which is already deleted
with retain snapshots option fails with 'EAGAIN: clone in progress'
error. After subvolume deletion with retain snapshots, the subvolume
exists until the trash directory (resides inside subvolume) is
cleaned up. The subvolume deletion issued while the trash directory is not empty, should
pass. This patch fixes the same.

Credit: Issue discovery and fix suggestion to John Mulligan <jmulligan@redhat.com>
Fixes: https://tracker.ceph.com/issues/54625
Signed-off-by: Kotresh HR <khiremat@redhat.com>