rbd-mirror: unbreak one-way snapshot-based mirroring

rbd-mirror: unbreak one-way snapshot-based mirroring

Snapshot replayer needs the remote's mirror peer uuid to find its
snapshots in the remote image.  It is obtained by listing remote's
mirror peers but RemotePoolPoller::handle_mirror_peer_list() skips
tx-only (MIRROR_PEER_DIRECTION_TX) peers.  In effect only rx-tx
(MIRROR_PEER_DIRECTION_RX_TX) peers are considered for matching
and snapshot replayer always fails with "failed to retrieve mirror
peer uuid from remote pool" error.

Instead, skip rx-only (MIRROR_PEER_DIRECTION_RX) peers as we are
definitely not interested in anything having to do with mirroring
_to_ the remote cluster.

Fixes: https://tracker.ceph.com/issues/52675
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>