librbd/mirror/snapshot: avoid UnlinkPeerRequest with a unlinked peer

librbd/mirror/snapshot: avoid UnlinkPeerRequest with a unlinked peer

CreatePrimaryRequest could create some UnlinkPeerRequest with an already
unlinked peer in a scenario where you have multiple peers. This request
will not remove the peer (as it's already not linked to the requested
peer) and will skip deletion of the mirror snapshot if another peer
remains. Eventually the code will go through an infinite recursive loop
between CreatePrimaryRequest and UnlinkPeerRequest and segfault.

This commit adds an extra condition to make sure to not submit a
UnlinkPeerRequest if the peer is not linked to the current snapshot. If
there is already no peer in the list it will submit a UnlinkPeerRequest
to remove the snapshot.

Fixes: https://tracker.ceph.com/issues/50439
Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@cern.ch>