librbd/mirror: unlink peer might recursively loop

librbd/mirror: unlink peer might recursively loop

If the mirror peer set is (incorrectly) empty, it's not currently
possible for the unlink peer state machine to properly delete the
snapshot. This can result in a recursive loop between the create
primary snapshot state machine and the unlink peer state machine
until the stack depth grows too large.

Fixes: https://tracker.ceph.com/issues/48525
Signed-off-by: Jason Dillaman <dillaman@redhat.com>