rbd-mirror: return after finishing on unsupported mirror mode

The mirror-mode switch default in handle_get_mirror_info() calls
finish(-EOPNOTSUPP), which deletes this, then breaks and falls
through to the common code, dereferencing the null *m_state_builder and
freeing the request a second time.

Fixes: https://tracker.ceph.com/issues/78043
Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>