rbd-mirror: fix strict weak ordering in PeerSpec::operator<

The mon_host tier branched on "mon_host < rhs.mon_host" but returned the
same comparison, so mon_host > rhs.mon_host fell through to key, both
a < b and b < a could hold.

Only Mirror::m_pool_replayers reaches operator< (via std::pair key).

Fixes: https://tracker.ceph.com/issues/78049
Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>